You'll find 3-4" foam at places like Home Depot, hardware stores, marine supply outlets, and upholstry shops. The latter may also have scraps more than big enough, cheap too. 1" scrap foam can also be had at carpet outlets, 3-4 layers on each side of the case should work. Lay the top layer out and trace your guns on it, cut out to inlet, or leave whole. Just glue all layers in when you're done inletting.
Keyman (non sibi sed partiae)