This is an add-on for a spring clamp that I use to hold my fishing rod.
1x nylon spring clamp (that looks like the one in the picture)
2x M3x20 screw (ISO 4762)
2x M3 nut (ISO 4032)
You need to drill holes through the clamp.
I got my spring clamp from the local hardware store. There are several spring clamps on the internet that look extremely similar to the one I use, but I haven't tested any of them.
Woolee1 said the Pittsburgh 1 3/4" nylon spring clamp from Harbor Freight works with this model.