So, in other words, something that tapers at the bottom of your thumbhole to match the shape of your thumb, so that both the top and bottom of your thumb are snugly touching something? My mom used to use a rubber piece designed specifically to do that, I think it was called a ThumbUp. (For what it's worth, after she took off a summer from bowling to let some tendonitis in her balance arm heal, I suggested she try going without it and see if she still knuckled the ball. Since she got out of the bad habit over a summer of non-bowling, she doesn't knuckle it anymore)