For the Tip version it's quite simple to make your own . . . If I can remember (been a while) you need rivets for a pop-rivet gun. . . if you look at these items there is a shaft that can be removed leaving you the barrel, which is the part you need. Get a drill bit slightly smaller in diameter than the rivet barrel and upon finding the point on the tip that seems to be right for you, drill the hole.
You now slide the barrel into the hole with the flange side facing out. You can secure this into position using super-glue or clear epoxy and do the same thing with about 1/4-3/8 inch of pencil lead or the grease pencil type lead.
If you want a ink writer, you'll usually have to run about an inch long section of the ink tube with tip in-tact, through a small hole that will be higher on the tip, towards the nail but back along the "fleshy" side. Super-glue the the copper colored part of the pen into place but be very careful no to get glue on the tip itself. The ink tube can be glued down but I will usually use either one end of a band-aid or a piece of flesh colored medical tape to lock it into place inside the tip. It is wise to use a bit of white glue on the opened end of the ink tube so as to seal it closed. I've also used candle wax and just dipped into the wax a few times so as to layer it slightly and then allow the flesh colored bandage/tape to cover over it for safety sake. An ink writer should last most people, even if you rely on it heavily through the day, at least a few months to a year. You do however, need to keep an eye on the writing tip in that they clot and dry up.

I do believe there are images of my tip styled writers somewhere in the older postings either here or at the Magic Bunny. I think they have my banjo pick designs as well (which is the same basic process except they cost a lot less than buying a thumb and most banjo picks already have the holes in the right position.
