I like large roomy thumbpicks, and have tried a lot, in the large size. I assume you have tried the mediums? I have found National and Dunlop to be the roomiest, and the Ernie Ball, and Fred Kelly to be the tightest. So assuming that holds true on the step down to medium, maybe try the latter 2. Also, the Fred Kelly Bublebee ones, I couldn't even get around my thumb without cutting off circulation, but I found all of his to be tight.
This guy is from Megadeath, I think. He designed this pick clip thing. You put your own flat pick into it, and the metal part goes around your thumb, and you can bend it to fit. I tried one, and also thought it was tight, even with bending, and for me a little uncomfortable, but for a smaller thumb it might work. They are pricey though.