Hey Bill
you know that plastic/board type stuff used to make political signs (vote for John Johnson! Type signs) that you see during elections? I bout a MASSIVE sheet of that material and cut it into several sheets the size of the tank bottom. Then I sealed in the first layer and sealed the others on top of that. So far it's holding up beautifully. It's very strong but still allows for flexibility. Plus it's completely waterproof.
i would have liked to use plexiglass but at the time it was not something could afford. Turned out to be far more expensive than I thought.