Honestly, the most satisfying solution I’ve done in a metal panel is to drill a small hole, very cleanly, then heat the panel with hot air and smoosh a hot glue stick (the milky white kind) into the back so it melts through the hole. Let it cool, then carefully use a scalpel to trim the excess from the front. Finally, run hot air over the front briefly just to smooth the cut surface. This produces a lovely diffuser effect for a PCB-mounted LED, with a wide viewing angle.