I would`nt risk using anything veneered or varnished due to the risk. I`m also looking into how I could construct some kind of `L` shaped shelf for my gabber rex tank and thought about either using untreated pine, or a large fiddlestic play logs and perhaps balancing this on some pine blocks? Then having a smaller fiddlestic as a ramp. I hav`nt came up with anything much more ingenious at the moment!
Definately would`nt use old bits of wood from furniture though. I know some people have made cages from old cupboards and such, but the wood is mainly on the outer shell and not actually chewable.