much like a cube! my tank is all glass 30x21x24 and it uses the thickness for 30 gallons (it's 60 now i think). a thicker glass would be more safe. (1/2"

?
How are you planning to seal the points where the glass and plywood meet? Marine Epoxy? What if the wood "warps"? will you use a paint sealant (those paint which waterproof anything?)? Or paint the inside marine epoxy?
what about a cement and glass tank? use cement slabs instead of glass?