Sorry, don't know how to edit
I didn't see this when I posted last time. I wanted to elaborate on what I mean by PVC liner. It's sold as a wide sheet of PVC that's something like 3mm thick. I would be caulking any and all jointed areas with silicone if I did it. However, for peace of mind, I think it's worth looking into alternatives so I don't put a bunch of work into something that turns out to be useless.
I didn't know about purely PVC-made structures, but I doubt I could build them. If I go that route, though, is there any good tutorial for how to do it?
Thanks again.