One challenge of fastening to the aluminum channel is that it doesn't work well with regular t-track bolts, which can spin freely in the channel.  I'm experimenting with methods of fastening - bolt in the knob with a nut in the channel, and nut in the knob with the bolt in the channel.   I like the bolt in the track because the same bolt can accommodate different thicknesses of fixtures.  The problem is that it's hard to find purpose-made bolts that both fit the channel without rotating and are long enough to extend up through the 3/4" thick spoilboard and the 3/4" thick fixtures I'm currently building. 

I've been making my own by cutting down the head on "elevator" bolts - some sort of leveling foot I'm guessing.  They have a large circular head that can be trimmed down to a shape that will slide into the channel.  It's a bit of a pain to do, but it works.

I score the top where it needs to be cut.

Clamp it in the vise and rototool away.

A few passes with a file clean up the sharp edges.

I grabbed a scrap piece of BBP that was less than an inch wide to make some wood wing-nut style knobs for the clamps.  This was the first test of the clamps, and they worked great.

I cut out a single knob to check the fit of a threaded insert.  I had to drill out the center hole a bit wider to get the insert to fit properly.

With the design modified with a larger center hole, I setup to cut out 3 more knobs, with the clamps positioned so the bit would miss them.

But mid-cut, I realized that the spindle carriage was very likely to clip the knobs (there's one con for having the knobs closer to the work).  I paused the job and re-positioned the clamps (with a sacrificial shim), then completed the cutouts with no incident.

I'll need to make some lower-profile knobs for these clamps, but I like the overall shape of these knobs.  I can really get some good leverage on them, so I'll probably use the same shape for the replacements.

[next day]

I made some lower-profile versions of these knobs and tried 3 different styles of threaded insert. 

The T-nut was a bust - the prongs split the wood since they landed so close to the edge.  Plus, I didn't like the idea of all that extra metal being exposed and attracting a carbide bit like a magnet.

I also used the same threaded inserts as I did in the taller knobs.  This does bring the insert up to the surface, but only because the knob is shorter.  I could settle for this option.

Finally,  I made a hex nut recess in the bottom of the knob, scuffed-up the nut with some sandpaper and CA-glued it in place.  I was careful to keep the nut flush with the surface of the knob so that as its drawn tight to the clamp the bolt won't pull the nut out.  As long as the CA glue holds against repeated tightening and loosening, this is the best option, with the smallest chance of a wandering bit crashing into metal.