When I was buildng boats, you could document the vessels at local documentation centers. Fortunately there was one in Long Beach CA. I became familiar with the requirements--and they have not changed. We did rout numbers and epoxy the teak board onto glass.
For the C Dory 25, I used vinyl stick on numbers (actually my son put numbers on and did a great job). The numbers were placed under the table on the side of the hull. I roughed up the gelcoat with some #80 sandpaper, and used quick set epoxy from West Systems, to layer on a single coat of 6 oz cloth.
I used my label maker to put "Doc no, XXXXXXXX Frequent Sea, 25 foot C Dory, white hull, green trim" right by the microphone on a label about 1" x 3". I believe that there is on convenient area for the amount of room needed for the number near the radio.
The area on the hull, under the table is easily seen, but does not detract from the interior decor. Sorry I didn't take any photos, and the boat is 2100 miles away. But I have done under the table, on the hull on several boats, and it works very well.