My cure for this is always the same. I take gray packing foam (the softer kind that PC parts usually ship in), cut it into appropriate sized chunks, and then STUFF the chunks. It's an art. :wink: Lightweight, doesn't hurt a thing, easily reversible, and if you want to add a bit of weight you can just pick your spot to add the weights and pack it on in with the foam.
With this method I deaden that hollow sound and give the recoil pad a little something to butt against.