You can put the recreational use registration number anywhere that is accessible without tools, even inside the battery compartment if you wish. I printed the number on white paper, trimmed it tight, and just use Scotch tape to adhere it for a neater look than using a sharpie. You really don't see the tape or white paper against the white shell. For the commercial registration number, it needs to be easily visible so I printed it large and adhered it to the outer side of the rear prop arms. For that, I got a little fancier and printed the numbers on some clear adhesive film I had at the office.