I can fill most voids with denser grades of Styrofoam cut oversize with a razor sharp blade & squished in. It's particularly good in magazine cutouts & the corresponding void in stocks.
I use modeling clay and the little foam ear plugs. The ear plugs are great for little holes and other things. Just cut a small piece off compress and put in hole. It'll expand and keep the bedding out of that particular area, plus much easier to remove when done.