Corrugated plastic sheets, like they use for yard signs.
Home Depot carries it in two sizes and it is in the same display rack as the acrylic and glass and it's less $$$.
It is easy to cut, easy to drill, is rigid but flexible and bonds extremely well to glass with silicone.
I've used it to make internal sump/baffle filters for my two ten gallon tanks and have no complaints. (couple pics in profile)
Can't go wrong with it.
tip: to form it so it has nice crisp corners, simply cut through only one side on the sheet, fold and there ya go. (you will understand if you use it)