Personally, I'd give them a call and see what they'd charge to sell you just the foam insert. Seems like your case is about the same size as the 8-pistol and all you'd probably have to do is trim some of the edge material.
Really just breaks down to cost and what you're willing to pay as far as what the possibilities are.
If they won't sell you the insert, the easiest route would probably be to purchase several sheets of either the the less dense "Pelican-style" foam or the more dense pool noodle style foam. You could then draw out the patterns you want and cut each sheet piece separately via foam cutter (either hot wire, electric knife or whatever) and then glue and stack them together in the arrangement you need. Kind of like a big foam 3D printer.
Really just breaks down to cost and what you're willing to pay as far as what the possibilities are.
If they won't sell you the insert, the easiest route would probably be to purchase several sheets of either the the less dense "Pelican-style" foam or the more dense pool noodle style foam. You could then draw out the patterns you want and cut each sheet piece separately via foam cutter (either hot wire, electric knife or whatever) and then glue and stack them together in the arrangement you need. Kind of like a big foam 3D printer.