i have thought about opening up a thin, synthetic quilt & shooting in some extra down that was left over from trimming down sleeping bags to convert them into quilts.
seems like at least part of the down would stick to the synthetic insulation, but i think that would be OK. the rest of the down might be some what "shift-able"???
it's bound to make it warmer. wouldn't hurt to try. it might not be a bad combination.
don`t leave the CREATOR out of the creation!