Wada Cello Bow Grip Aid
Designed by a cellist, the Wada Bow Grip helps students nurture effective relaxed bow holds. With a malleable, snap-on design, it is suitable for cellists of all ages and sizes.
always on the lookout for a better mousetrap, I tried this. It does relax the thumb, but's a bit bulky and heavy, so I think it's best for discovery purposes, then return to a revised grip. I found the Stringvision more useful.