Is there a material, that can be shaped in a custom shape, and then harden?

I need to make 2 half-spheres, with some minor modifications, so what material would be ideal to mold into a half-sphere, and then harden, so it would become hard?

I don't know how it works but I know it doesThey even have laxatives that are a powder that you mix with waterThat's like MAXIMUM fiberRead the ingredients or call the 800 number and ask them.

How big? Makes a difference.