If your thumbnail damage allows, you could maybe try using the larger size Alaska Pik - the sort that fits under a minimal natural thumbnail, and provides a sort of extension. (Click on that link for a picture)

It's much more like playing relying on a real thumbnail - the angle at which you hit the strings is exactly the same. You can file them down to whatever length feels right. They are the only thumb or fingerpicks I can manage to use - you get the same kind of feedback to the finger and thumb as you would if weren't using them, which isn't true with any other sort I've tried using.