I know they're cheap and I could just order one, but I need it today, so I gotta make one. 1/2" 5/8"? 3/4"? I lost mine so I cant remember. Or do I just guess & shim?
Guess and shim...the important thing is that it is not rigid mount. Use what you have...cut and old tire and use the sidewalls for roughly 1/2 inch, or tread for thicker.
The ones I bought recently were more like 1/4". I think I will go the tire route and make some as the new ones just don't seem adequate.