I'll see about snapping some pics of my thumb ring when I get home tonight. I made mine out of PVC, I cut it right out of the side of an old pipe, the pipe was maybe 3" across and probably 1/4" think, maybe. I wanted it a little thicker around the hole so the string wont bite your thumb, basically the old vermil style, so I cut out the long oval shape and then a circular shape as well. I used CA glue to glue the circular piece to the "top" of the oval and then drilled a hole in the center and used one of those cone shaped rotary grinder heads to enlarge it and made a slope towards the tip on the inside. It's very easy to customize the fit this way as the PVC is relatively soft material and sands/grinds away well. Once it fit relatively well I put it in the microwave and heated it up until it was fairly pliable (use tongs, lol) and used that to put a curve in the tip (until this point the whole thing was basically straight perpendicular to my thumb except for the sloping in the hole). Then I just reduced the outside dimensions, rounded the circular "top" of the ring, rounded the edges on the tip etc. I made about four or five before I got this perfect one but it was the first four or five I've ever attempted and I'm not real great with my hands. Also the first one with this double thick design was the perfect one so the others may have sucked just from the thinness of the ring. One of the cool things with PVC is when you heat it up in the microwave it gets kinda tan with browner highlights and looks just like bone, I've had so many folks ask me what kind of bone it is, lol.
Edit: I found one photo I already had...