Yesterday evening, I cut out the parts for the long version of the clamps and got one assembled.
On the first test, the top of the plunger snapped when pressure was applied by the cam. I thought that the hardwood plunger would have been stronger, but I guess that is a narrow piece of end grain.
My fix was to cut off about 1/4" of the top of the plunger and replace it with a cross grain piece. For future clamps, I'll adjust the design to incorporate a wider cross grain piece.
I preemptively applied the same fix to the plungers for the stubby version. It's a good thing I went with a design that allowed it to be disassembled.
The second test went much better. I will probably add a patch of sandpaper or some other non-slip material to the clamping pad for good measure.