The "Super Clamp" (manufactured by Manfrotto, et al.), is widely used by photographers and stage hands for mounting cameras, lights, audio and other equipment. A triangular insert is supplied with the Super Clamp. The insert is used when clamping to a flat support, such as a board, etc., however, these inserts are easily lost. Here's a replacement for the Super Clamp insert.

Although this part doesn't look exactly like the original, it is perfectly functional. I printed in PLA, but ABS will work just fine.