For quick and dirty custom foam forms, wrap the gun with food wrap. Make it many layers so the covering is at least a quarter inch thick everywhere. Get a box big enough to hold the gun with an inch or more around every dimension. Buy some canned expanding foam at Lowes or Home Depot. Cover the food-wrapped gun with petroleum jelly. Cover it thoroughly. Lay down a base of foam and let it cure enough so you can lay the wrapped gun on it. Carefully foam around the wrapped gun so the foam expands above the top of the gun and curls around the top a little. After the foam hardens, using a sharp serrated knife, carefully cut away the foam enough to remove the wrapped gun. Then cut vertically around the inside edges of your form.
You can line the inside of the form with cloth, ultra suede, or whatever you like. just make sure you cover the gun with many layers of plastic food wrap, and then completely cover every inch with petroleum jelly before using the expanding foam.