Someone posted how to make a glass top out of plexi-glass a few months ago, but I cant find it. If you or know someone who made it, please let me know and post it back on here.... that would be great.
I know this isn't what your looking for but I found if you not placing anything on top of it you can just use plain old plastic. I'm not sure what tank you have so it may help you if you say what size tank you have and some others may be able to help.
Most of my tanks just have a piece of styrene cut to fit over the top. Minimizes evaporation and condensation in the fishroom. Lets in plenty of light, eliminating the need for a light over each tank.