The Sea-N-Sea clamps worked great, but they had some downsides:

A simple cam clamp would be a minimal solution.  The idea was to have a moveable dog that could be fastened along the track, and the cam clamp would pivot on the anchored dog.

My first attempt was a plywood disk for the dog, a plywood cam lever, and a slice of 1" PVC pipe as a bushing (similar to the hub of my flip-top cart).  To fit the PVC pipe within the cam with enough wall thickness on the plywood, I had to scale-up an earlier cam clamp design.

I had sized the plywood dog based on the nominal inside diameter for 1" PVC pipe, which turned out to be a little too loose for the section of pipe I used, so the fit was a bit sloppy and the clamp didn't hold tightly.

Also, with the increased diameter of the spiral, clamping pressure was achieved with a smaller rotation.  The limited throw also seemed to make them easier to work loose.  It might have been the plywood cam against plywood work, but the cam tended to rotate the work rather than clamping it against the fence.

For the next iteration, I reduced the cam lever size, eliminated the PVC, used a section of 1" wood dowel for the dog, and cut the cam lever from MDF.

I counterbored the dowel to drop the bolt head below the surface.

The smaller cam spread the horizontal travel over a greater rotation of the lever.  And the MDF didn't grab the work the way the plywood cam did.

I thought that the wood dowel might not hold-up over time - with the pressure of the screw, the end grain could shear at some point.  I bought some 1" diameter copolymer rods on Amazon for the dogs, and they'll hold up much better.

I like this simple clamping solution, so I'll make a set of these.  There's a couple things that are not ideal:

I'm pondering a variation of the Sea-N-Sea clamp that shifts the knob closer to the work.   That's up next.