I can tell you that a 1" thick piece of laminated glass (2 pieces of 1/2" laminated together) WILL work at 10' long by 4' tall. I second John's suggestion on concrete. Just don't use sanitred to seal it. 
If you can afford cell cast, then do that. If you do glass, I still stand by recommending laminated glass. Laminated has a film between the two chemically bonded pieces of glass, so that if it fails, it doesn't fail catastrophically. Think of a car windshield.
Good luck. I'm over a year into mine, and it isn't even close to being done.
If you can afford cell cast, then do that. If you do glass, I still stand by recommending laminated glass. Laminated has a film between the two chemically bonded pieces of glass, so that if it fails, it doesn't fail catastrophically. Think of a car windshield.
Good luck. I'm over a year into mine, and it isn't even close to being done.

