Very nice work as usual, Nigel. I have some of the commercial ones and find them very useful.

As your design evolves, you may want to consider V-grooves in the jaws. There are many useful options; parallel or perpendicular to the main screw, or both. Grooves in both jaws are nice but even if only in one jaw are still useful when clamping things with a rounded profile.

Clamps with interchangeable jaws are another design option. A shoulder bolt and nut in place of the rivet would make interchanges easy.

Plastic jaws and, dare I say it, wooden jaws are also a possibility for avoiding marring the clamped object.