I used an ultra fine point Sharpie marker and just wrote the number directly on the drone. Inexpensive, permanent, and very lightweight.
While I was at it, I wrote my phone number on the drone, figuring that if the drone flies away and gets found by a stranger, the phone number is more likely to let them reach me than the FAA registration number.
Actually, the Sharpie is not totally permanent. When I changed registration from recreational to Part 107, I was able to erase the recreational registration number with denatured alcohol, a Q-tip, and a few minutes of effort.