Should work. You could put concrete or rock hard water putty in it maybe. Would sculpy baked be waterproof?
I think sculpy, when baked correctly is considered waterproof. We have some kid-art sculpy that has seemed to become brittle over the years and a little more dull. But looking at the wisdom on the internet, I guess the problem is that it was not baked long enough or at a high enough temp. (The opposite of what I thought.) I also thought that a sealer would have helped slow this aging, but I haven’t found anything to back that theory either.
Here is one site with info:
There are probably many more.
Looks like a fun mold!