I built hoppers out of 1/2 mesh that hang on the side of the tanks (for the rats that are in tanks anyway). They measure 9 squares wide and 3 squares deep. I have a top on them that is only attached on one side that I can flip up to fill. I started off using a twist-tie to keep it closed, but the rats would chew through it. They weren't smart enough to open the top and get food that way, so I just left it.