I have found a very inexpensive way to make my own thumb breaks. Mine I simply use 12" hose clamps and use the unthreaded section of the clamp. It can be cut with tin snips to any desired length and can be drilled with any sharp drill bit. It is stiff enough to hold up to repeated use but can still be bent or straightened to shooters desired position.
It works well for me and so far my customers love it.