Fiber glass will work, but only once it has dried out, the only thing that you will need to do is build it in two half! and then make it one unit, reason I say this is that you will have to make the inside smooth either by sanding it or by by painting some extra layers of resin to make the inside smooth and safe for the spiders, as fiber glass can be sharp and and dangerous for soft bodied creatures.
Unless you will be making mold and pouring resin into mold this will be safe as the resin will be smooth.
Also to paint fiber glass is also a Challenge.
But once it dries it is safe to have in your enclosure!
What about baking the clay, wont that help it from getting soft, also what about sealing it with a water proof paint??
