You need to break out the Yellow Pages and look up "plastics"... call one up that seems near you, and ask if they sell AcryLite to the public. Most do. You want acrylic, not plexiglas - the stuff Home Depot sells. Plexiglas ages, turns yellow, gets brittle, and cracks. Acrylic is harder, and you can buy various thicknesses.
For an overflow box, 1/8" is ideal. I built mine out of clear acrylic, and I can see if any problems develop and make sure all is well. Being clear, I see when a fish or a crab or a snail climbs in (all three have), and I've seen copepods walking around in there (which was cool). The BAD thing is that it loves to grow algae in it, and it is rather annoying to clean out. I clean it about once a month, and pull out some thick matted hair algae that definitely restricts the flow.