And you took the words right off of my keypad! I had JUST thought, "oh, wait...KIBBLES!"Any grain in a bag should work well - even flour, salt or sugar would do the trick! (though I'd probably double bag those just in case - the mess if they split doesn't bear thinking about).
Thinking about it, if his kibble is pretty small even that might work for this purposeYou just need something with a bit of weight that will mould around him