I built one for my girlfriend's compass (finally got her off the stupid "consoles" and on to proper wrist-mounted gear.)
I ended up getting an end cap for ABS pipe (black) in the appropriate diameter, and turning it down to the depth required, as well as drilling the appropriate slots in it.
I then coated the thing in "Tool Dip", which is liquid rubber, and smooothed the whole thing out, as well as gave it some grip, which the slick plastic wouldn't do. I added three countersunk SS screws to retain the compass, and I was done.
It worked pretty well, but was kind of bulky.
I think if I had to do it all over again, I'd just use some marine grade epoxy and some fiberglass I have lying around, with a suitable coat on top of it to protect it from UV (maybe even the tool dip...) It's quick, cheap, and easy, and a lot less trouble than trying to shoe-horn things into a pipe end cap.