I don't know but Battlefoam and KR Multicase are the two companies I thing of for miniature foam cases or inserts but I don't know if they have anything that would fit correctly in a GMT box. You could always poke around their sites or give them a call though.
Won't the foam stick to the miniatures? Unless it is closed foam, I would be worried that it would off-gas and destroy or stick to the painted miniatures. That sometimes happens with model trains, my other hobby.
I clicked on this thread because I thought it said "foam pudding". I was thinking, what in the world does foam pudding have to do with gaming? And what flavour is it? Needless to say, my eyesight is not the greatest.