Grlllsfly.. If you go to Lowes or home depot and to the heating and cooling section. Most stores have the fairly dense black insulating foam. It works well and cuts nicely if care is used.
Also most stores that carry fabric for sewing will carry foam for projects like cushions etc.

I use an old, old Bell telephone tool kit. Its metal and covered in leather and has a panel in the lid that flips down to hold small scale tools. I purchased foam, cut to fit and it works well.