Topping off around the clock is the best way to keep salinity stable, and doing so from a reservoir avoids any risk of excessive additions of RO water.
The ATO reservoir measures 22" x 15.5" x 32" and is made with 3/8" cast acrylic. The only opening is a sliding access door at the top corner, just above a float valve. The container holds RO/Di water and I am very careful not to let anything pollute the water since cleaning it out would be extremely challenging. The 150gpd Boosted RO/DI system feeds water directly into the reservoir so I never have to lift a jug of water ever.