So it was a combination of two things:

1. M4 socket head screws (they were long, maybe 30-40mm). These are the green arrows in my screenshot above. I did maybe 5 or 6 of these. The extrusion itself I got counterbored twice on both ends, but then I tool a drill and made my own holes through the rest of the extrusion for these M4 screws. I think I used a regular washer up top, but then on the underside of the tabletop I used a locking washer.
2. And then the red arrows are SPAX all purpose screws, maybe 1” or 1 ½”. I just drilled those right into the desk.

Here are a few more drawings and pictures regarding the socket head screws:

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