I have worked with plexiglass. it is very brittle and flimsy and does warp. you would be happier with 3/16" acrylic. a sheet of that costs about $100 from a plastics distributor. Then you have to join the cut pieces together with a bonding solvent and make sure it is perfectly square. In your case, I would just look at a larger Exoterra tank. they make them as large as 36x36x18. that should be large enough. They cost about $400. plus it will look a whole lot nicer than a mod job. I have also seen larger tanks for less that were not Exoterra and still very nice. good luck