When I had a filter on the lens, I was constantly losing the original metal lens cap. I now have sort of permanently, a light yellow filter (40.5), a Heliopan Digital (huh?) cylindrical metal lens hood, capped by a 42mm inside diameter Schneider slip on cap that has 150 Xenar scratched on the inside. I've got my filter, my lens hood, and a lenscap that doesn't fall off and roll away. If I hadn't found the slip on cap, I would use a metal screw in. The outer end of that hood is also threaded for 40.5.