When I used to use thumbpicks I'd take the big National white one and plunk in a pot of hot water (not boiling,if it gets too hot it will straighten out the pick immediately) for a couple of seconds pull it out, put it on my thumb and run cold water over it while holding it in place and I'd have a thumbpick that conformed to my thumb perfectly. Then I'd thin it out on the top side of the lower flat part with a piece of sand paper ( Number 6 suggests using a Dremel for this , which sounds like a good idea). I liked it at about the equivalent of a med. flatpick. Next I'd round it off and shorten it a bit so it just came past the edge of my thumb. It's something you can tweak to your own specs.vl