Good idea but....
What's the weight of magic scuplt like? I have some different types of epoxy putty and while not heavy, they're noticably heavier than foam. Other benefits of foam is that it can friction hold the items and it can cushion impacts.
I've done some cases with multiple layers of foam and then covered with felt (OK, I used thin foam as I didn't think of felt!) but the problem I ran into was it's not as finished/polished looking as I'd like, too many wrinkles.
I guess what would work would be some sort of stretchy felt. I wonder if there is such a thing? Maybe I should just use velvet as I know that has some stretch/give to it. Oh, thinking as I type, I remember a loooong time ago using some powdered velvet for a wood jewelry box (apply glue and then sprinkle with the velvet "dust" and then, once dry, vacuum up the loose stuff. It worked pretty well and would be great on the putty or any hard surface!)
Don't get me wrong, the harder insert definately has it's place, but I'd like to have the easy ability to do BOTH!