Pond liner is superior over anything else, if you can afford it. Other tipes of lining like shower curtains work great but you have to replace them eventually. Pond liner is indestructible. It lasts forever
I use pond liner because a digging tortoise isn't likely to tear through it if they get down to it - but it is more costly than some other options.
Make sure the raised base that this will be on is solid enough to support the weight of the new enclosure when it's done and filled with well watered substrate.