just tried this and it worked out pretty well. i now have zero wiggle between receivers. basically its just a thin rubber band between the upper and lower. i super glued it into place and trimmed the ends. while it was drying i assembled the upper and lower and pulled on the ends. this was important because stretching a rubber band also makes it thinner. i didnt want to leave the full thickness of the band in there because it would put too much force on the receiver pins. after the glue drys the band remains stretched (thin also!) then i trimmed the edges, and cut down some weird folds in the band caused by the glue drying. now it only requires a little force to close the receiver, but everything locks up nice and tight.
basic positioning of the elastic band
how it should line up on the upper
final result. this is after super gluing and trimming
and yes i also polished the bolt where the hammer bearing rides. racks a lot smoother now and sounds better too.