Bob

I never acquired the synthetic Omega. I'm not sure if you can get that rubber foaming in there. There may only be small openings once you remove the pad.

I'd suggest measuring the hollow cavity with a retractable measurer. Buy a couple of 3/8th bolts about 1-1/2" shorter than you measured. Slip the bolts in .. then shoulder for weight gain. Now either add bolts or remove one. Go buy a spray can of Great Stuff poly-foam with long nozzle. If you can, tape the bolts... if not.. no big deal. Spray until the bolts are covered. Use a thin piece of cushion foam to make-up the gap difference between the Great Stuff and recoil pad.