What I should sculpt with?

I’m having some serious issues with the idea I showed You in my last note.

As I told You already, I’m not sure what material I should use to prepare base.

I’ve been thinking about using Sculpey, but to make such big base, the cost would be way too high. I also found Milliput putty, which also could be used to sculpt, but it’s expensive as well- making base for those two figures would cost as much as buying one new kit. Typical modeling clay, which I can afford, is way too fragile.

I’m looking for any help in finding less expensive modeling material, which will be durable/stone-hard after drying.

Any ideas?

Take Care.