So I read people saying to use a shower curtain or something of the like to line their indoor enclosures. I have 2 questions. 1) what is the liner for? 2) do you nail/staple the liner in or just place it?
The liner is so that the wood doesn't rot. The substrate in a tortoise enclosure should always be humid. It should be damp to the touch but not so wet that you can squeeze water out of it.
The liner needs to be fastened in place or the tortoise can get underneath it, and that would not be good. I have found that with larger tortoises that like to dig, they will destroy the liner quite quickly. For them I use Thompson's water-seal (the non-water based kind) and then caulk the seams to protect the wood.