I made a thumb ring from leather which it fits snugly on my thumb and then goes into the thumbstrap. It gives a little extra cushioning and when my father in law comes down from Canada, who has bigger thumbs than me, he plays it direct without using my custom thumb ring. The other benefit is that where the ring is a snug fit on my thumb I don't get sore after an hours playing as it is leather against leather (as opposed to leather moving against my thumb as I go up and down the buttons. I made the thumb ring by measuring the circumference of my thumb with a piece of paper, cutting out a length of leather equal to the circumference with the same width as the thumbstrap and stitched it together.
Stephen