“Done it myself. Legally you can make one, just one. My wife has to make her own, I can’t do it for her. Once made it cannot be sold or transferred it has to be destroyed to get rid of it.”
What happens if I lose it in a boating accident? Can I build another one?
Nobody, but you is keeping track. . . there is no limit in the law. You can make as many for your personal use as you like. You don't report it to anyone. Polymer80 sells the 80% lower complete with jig, milling bit, and the two drills you need to complete the lower milling for as low as $59.95. You can complete one on a drill press or, if you are very carefull, in a vice with an electric hand drill, but I wouldn't advise it. Some have done them with dremel type hand tools, but that's only for the really patient. There are YouTube videos showing step by step how to do it by both means. They shoot well and are reliable.