This is an example of a vertical LEFT corner internal overflow box. Normally these are built 8" wide x 5.5" deep (front to back) and to your desired height. Custom sizes are best, so be sure to give me your exact inner measurement from the bottom panel of the tank to the underside of the plastic trim. I'll be happy to give you a quote.
Pricing starts at $65.
This is the silicone I recommend, found near the glues in the paint department at Home Depot for about $5. You don't need a caulk gun nor a huge tube of silcone for this project. :)
Using Aquarium-safe silicone found at your LFS or Home Depot, glue this overflow into the left corner of your clean dry tank. Let cure for 24-48 hours before water testing.
These are precision cut on a CNC machine to your exact specifications.