Personally, I don't see a need to design one when I was born with two perfectly good thumbprints... If you wanted it to be REALLY personalized, just ink up one of your thumbs and press it to the cleanest paper you can find. Scan that and stretch/skew/crop to your heart's desire. If you're worried about using your own print, you could manipulate it a bit, like flipping it or using one half and a flipped copy of the same half, etc. A friend's print will do as well - or a parent's, or a child's.