The good gray packing foam that is used/was used for delicate component shipping is good, as is an old foam mattress pad. Just cut it into chunks and stuff it in until you can't pack it tighter - you might want to use a piece of thick cardboard over the foambeneath the recoil pad so as to keep the foam from deforming the pad.

Great Stuff expands like crazy and would be a nightmare to remove. The packing foam chunks accomplish the same thing without the mess. Dave's method sounds pretty reasonable too.