Would a vertical sheet of 1/8" steel inserted inside the trap with rubber mulch in front and back of it do the trick? (I happen to have some 1/8" sheet steel) How far back to stop the .45s or 9mm? It seems that it would prevent the bulk of the mulch from wanting to drop and apply forward pressure on the front.
As a funny side note, the level dropped down enough to allow a hornets nest to develop in the top above the mulch. It made changing targets an aerobic activity for a couple of days until I introduced some wasp spray.