This is a good idea, but if the peanuts are too soft, then maybe another type of foam. Say, heavy foam for cushions? Or maybe some plastic foam for packing heavy, easily broken things? My computer came in such a box with form fitting foam. Wish I'd kept a little of it, might even have some around. This kind cuts easily with an electric knife or an hot electric foam cutter, sorta like a cheese cutter with heat.
Thanks for the idea, I'll try it.