Melissa, You can use plexiglass, thats how I made my 10 gallon sump. Glass would be better, because its stronger and wont bow like plexi will. Also, silicone adheres better to glass than to plexiglass. I just bought some plexi pieces that I measured to fit inside the tank, braced 'em w/ a wooden block and used some angle clamps...(find em at home depot) siliconed em inplace w/ aquarium sealant, and left alone for 48 hours like the directions said. Been working fine for me.
Nick