Building aquariums from opaque materials?

This is building on the "floor tank" or "indoor pond" concept. How thick would Plywood have to be to hold a 4-foot deep by 8-foot wide by 4 foot tall block of water? Steel? Other materials?

I had considered the possibility of using a swimming pool, but none will fit in the niche that I have available. Since this is still all hypothetical I would appreciate any and all comments and advise. The tank could potentially be taller, but for carp I wouldn't think this would be nessasary. Thanks.

Also bear in mind I may never do this, I just want some information in case I decide I want to.

Thanks for the help. I may actually build this thing (I want to keep my Koi, and maybe two or three others) but an outdoor pond simply isn't an option (maintainence is too big of an issue when you live in a woods!).