Most anything. Pond liner, plastic, marine paint, polyurethane. What ever you use, if it has fumes, make sure you let it air out before putting torts in. Don't forget pics of the finished project.
I usually use heavy plastic for my enclosures. The problem with it (and most other things) is that it works great when new. What ends up happening is that torts often try to dig into the corners and depending on the size of your tortoise they will slowly (or quickly) make holes, and your water proofing is no more. It usually takes a while though, and depending on how much water you add is often not that big a deal.
I was considering wood for my enclosure but I went with cheap slump faced cement blocks. I hammered rebar in the holes and into the ground to prevent digging out. I poured a few bags of dry post cement in the holes then just hosed in some water . No mortar between them. I could easily knock it down with a sledge hammer if I had to change it or extend it. But it's pretty solid. I also made the den the same way. No worries of a CHE catching cement on fire. The roof is tiled cement board on a hinge. I have heat panels on the walls and a backup CHE for winter time.
I LOVE the shower pan liner (like pond liner) that I put in my inside enclosure, folded up the sides (past substrate level) and stapled. It is very tough, (which makes sense given either one is meant to be exposed to water over years). They can't even make a scratch in it.
At Lowe's, it comes in 4 or 5' widths (I believe), purchase by the foot. Also in prepackaged bags (5'x6').
I have seen it on Amazon too!