I've used sugru for a couple of domestic repair jobs, and it is fairly stiff when set. It's also quite expensive. I prefer polymorph for a stiffish mouldable plastic. Blue stuff is sold as a flexible mouldmaking material, but could also be used for puppetmaking. Drawback of all of them is that they have a surface that won't accept paint very well.
Another product with possible uses for stopmo is self-amalgamating tape, which bonds to itself when stretched. Chandlers and builder's merchants will have it. It is more flexible than any of the others.