I guess I'm a bit of a dumpster diver as I will bring home cast-offs from work that appear to have a second-life value possibility. This foam is part of the packing materials that come with a microwave dish / radio setup that we deploy here and there when terrestrial infrastructure is unavailable or insufficient for our needs. Anyway I seem to have gathered quite a pile of them even after giving many away.
My file/rasp organization has morphed over time but, they still always seem to get jostled around which bothers me on some low subliminal level . A felt pen, a cheap razor knife and about 20 minutes of my time got me this out of half of one of the sheets of foam. This sort of thing isn't for everybody but, it was way simpler than I thought it would be and the materials are free. Just thought I'd post in case someone out there went "ah-ha! I knew I'd been saving that whatzit for something!".