I have both Match Technical and Lensmate thumb rests… Latter are much better designed, former are more solidly built from (arguably) better material.
If you want a thumb grip, you're on the wrong path. You shouldn't be using the hot shoe as a fulcrum. That "thumbs up" gizmo really is a thumb rest, an index point to aid with the grip. Like when you're climbing a very steep hill, you may grab onto some grass; it won't even remotely support your weight, but might help with the posture and overall stabilization. Same here.
Lelle Kidd reacted to Trenton Talbot in Be careful with thumb grips.