The previous plunger clamp functioned great, but it seemed a bit wide for what it needed to do.  For rev. 2, I slimmed it down.  Instead of the sliding plunger motion being constrained by a track in the body, I changed the design so it could slide over a inner "rail".  In this design, the inner rail needed to be stationary to make sure the plunger maintains linear travel.  This could be achieved by just gluing the rail to both top and bottom pieces, but that would mean making the plunger captive.  Before I went that route, I thought I'd try an option that allows the clamp to be disassembled.

I'll glue the rail to the bottom piece, but leave the top unfastened.  I added a indexing pin to the the rail, which will help prevent the unfastened top piece from shifting when the cam is tightened.

The end of the exposed plunger gets built-up with some cross grain strips so the clamping surface is 3/4" tall.

First test. 

I think this is a winner.  They're unobtrusive (only 1-1/4" wide) and they do the job.  Next time I would reduce the gap between the rail and the plunger.  I allocated a 1/64" gap on this prototype, but as small as that is, it still feels a little loose.  I'll half that for later ones. 

I can adapt this design for clamping perpendicular to the track by elongating the whole thing and changing the bolt hole to a 3" long slot (which is the distance between the exposed tracks).