You need to break out the Yellow Pages and look up "plastics"... call one up that seems near you, and ask if they sell AcryLite to the public. Most do. You want acrylic, not plexiglas - the stuff Home Depot sells. Plexiglas ages, turns yellow, gets brittle, and cracks. Acrylic is harder, and you can buy various thicknesses.
For an overflow box, 1/8" is ideal. I built mine out of clear acrylic, and I can see if any problems develop and make sure all is well. Being clear, I see when a fish or a crab or a snail climbs in (all three have), and I've seen copepods walking around in there (which was cool). The BAD thing is that it loves to grow algae in it, and it is rather annoying to clean out. I clean it about once a month, and pull out some thick matted hair algae that definitely restricts the flow.
Ideally, you'd want to build it out of black acrylic, which costs a little bit more (a few dollars at most), but if it is black, you can't tell if something is in it, if it is losing suction, or anything. If you could somehow build some type of shade to keep your lights off of it, it might help. I have a couple of pieces of black acrylic, and I've been thinking about making a shield to lay over it to help reduce the algae thing, only because it is a pain.
I don't remove it to clean it, I simply bend a flexible brush to fit, and work it back and forth. Takes about 20 mins once a month.
The plastic is bent by heating it over a propane torch. As it begins to give, keep applying heat and fold the plastic gently. Don't rush it or it can crack (break). Don't overheat it too much or the plastic will get a bunch of bubbles. Just take your time, and use your judgement when you think it is getting too hot. You can pull it away from the flame and let the plastic cool down for half a minute or so... It is really easy once you get started.
Here are the specs for you. I'll try to be specific, but ask more questions if you have any trouble understanding them.
You want the inner portion to hang down inside the tank below the water level. I like my water to be right at the brim, just below the plastic frame that caps my tank that the glass lids used to sit in. Then I added the additional height to get it up over the plastic trim. That total was 3". So 2" is underwater, with water pouring into the chamber.
I'm recommending you put the outer portion lower than the inner one, by maybe an extra inch. Mine works fine, but I'm guessing that extra fall only could help matters.
To assure there was enough flow, I made the overflow chamber (the upside down "u" area) 1/2" gap at all times. You might prefer 3/4" to assure volume. I think going larger would be detrimental to keeping the weir primed. Remember, that would be 3/4" x 7" wide.... which should really be plenty.
The main piece is built with 4 pieces of plastic. The first one is a 7" x 22.5". This is the breakdown, where the folds would go: 2" , 2.5" , 3" , 1" , 4" , 5" , 5"
I marked these with a Sharpie, but as I heated the plastic, the marks faded. I used a straight piece of wood as my surface to bend the plastic. After each fold, I put the piece on its side, to make sure things were still square. It was an eyeball job, nothing precise.
The first fold you have to make is the 1" fold, or center. This is the one that fits over the side of your tank, so you want that to be right. Then the other folds work nicely. One thing to watch out for is too much heat. It can make bubbles appear in the plastic, but it isn't a disastrous thing. I kept the piece moving over the heat - back and forth - as the plastic became pliable. As you make your folds, you might notice the very edge on the 90 degree angles flaring out a tad. If you can prevent this, it will make glueing the sides on easier. I didn't sweat this small stuff, but sanding can help take some of that off as well.
After you've made this piece, you need to make the inverted "U". That is a piece that is 7" x 9.5" The folds are: 3" , 2.5" , 4". I drilled a small hole in top (at what I considered to be the highest point where air might collect) and inserted a 1" piece of rigid tubing that I glued in place. Remember, it only goes in a tiny amount, as close to the inner surface area as possible so you don't have air collecting around this tube. Btw, gluing on this piece is the last thing you do, after everything else is done.
Your side pieces will be approx 9" x 6"... maybe a tad larger. I glued them on, and then used a router to cut them to shape. It was a 3/8" straight bit with a ball-bearing guide. The guide comes on the better-made (more expensive) bits, and will roll along the edge of the overflow box as the excess material is trimmed off.
I used Weld-On #16 to assemble the pieces. I glued the big "w" piece to one side, holding it firmly in place for about 30 seconds. Then I put my "U" piece in place visually, and traced the outline with a pen. After taking the "U" out of the way, I ran a bead of glue, and quickly put that piece in place. To do the other side is a little more complicated, because you'll have to trace both parts onto that plastic, run two beads of glue and assemble it quickly.
Give it a day to cure. Then test it in your sink. Fill water on both sides, stick a piece of airline tubing on the rigid piece, and suck the air out. Clamp down on the tube with your teeth or fold it, and watch your box VERY carefully to see if you have any air bubbles leaking into the unit, or water dripping out/off of it. If you do, dry it off and put more glue in that area. You should be able to pinpoint a problem area by visually inspecting the joints extra carefully. I had one problem spot that I reglued 3 times. Finally I took a large drill bit to that spot, leaving a hollowed out 'v' to fill it with plenty of glue. Problem solved.
Gee, I hope I've not scared you with all this information. It really isn't hard, just a little challenging. But I thought it was fun, and people are impressed when I tell them I made it myself. Makes me feel like a die-hard hobbiest. grin
The last piece you need to glue in would be a small divider. 7" x 2" tall in the outer section. This is important, as it is the only thing that keeps your weir from sucking air back into itself. Just glue it in place about 3/4" to 1" from the inverted "U".... please look at mine closely again if that doesn't seem clear enough.
The drain is a 1" bulkhead I bought from MarineDepot.com. (I drilled the hole in the center of the box. I think it was 2 3/8" - Your new bulkhead should be labeled to tell you what size to drill.) It is smooth inside, threaded outside. I was able to add that 90 degree elbow that you see in the next picture to avoid the slurping sounds. It just sits there - it isn't glued.
Like I said, it's pretty easy to build. I think it took about 30 minutes to bend and glue it together, after my pieces were cut out. That took maybe 10 minutes.
If you hate your first one, do like I did and make another one that is better than the first.
Due to a number of emails about this project, let me add a few more thoughts:
I don't make these for others. I made a couple for myself, because like any DIY project, it needs to be tweaked to my specific needs. If I can help you solve an issue, feel free to contact me.
I did make a black shield to fit over my weir snugly. There is no picture of it because #1) it isn't much to see, and #2) it's ugly. ;) You can't see it since it is within my canopy, but it creates the perfect dark condition to avoid any algae growth in the overflow section at all. I've not had to clean it once since the shield was installed over 6 months ago.
I switched the airline tubing to black. You can get it as irrigation tubing from Home Depot. Using black, it again prevented algae from growing inside of it and creating blockage. Where does it lead? It runs from the top of the overflow chamber to the intake of a MaxiJet 1200. I pushed it up through the basket/screen to always pull out any air that might accumulate in the weir and blow it out into the tank.
It works 95% of the time without issue. When for some reason I do get a ton of air in there, I pull off the basket, push the hose back into the intake, and use my finger to block the rest of the intake opening. This forces the pump to suck everything it can through my airline tubing, clearing out all the air in the weir. It is a lazy solution, but occasionally I have to do so.
How do I know when the weir has some unwanted air in it? When my surface isn't pristine and stuff seems to be clouding or floating there. At that point, I check the weir and do the above.
Here are some pictures of the weir being routed once glued, to give you some visuals of the process.