The only commercial one like the one Rick described above that I found usable was the Fred Kelly Bumblebee(?). Even so I went back to the standard Fred Kelly. At least for me, a pick point on the thumbpick similar to the point on my flatpicks makes things a little easier so to that end I do a little shaping with emery board and wet/dry sandpaper.
One of my students likes the Herco thumbpicks but they simply won't stay on my thumb as they open too easily.