To cut the slot out of the 2x4, I cut a 1 9/16" cut all the way
back 1.75". Then, I used a my jigsaw to rough-cut the larger
portion and finished it off with a chisel (see Fig. 3). Not easy and
takes some to get it to fit, but it works. Then, in the center of
the back of the cut, you need to chisel out a square hole *just* big
enough for a square 3/8" nut. This nut is what the thumb-screw
screws into. It's square so that when you turn the screw it doesn't
also turn the nut (the square nut stays firm in its square hole).
Finally, drill a 3/8" hole directly into the center front, through
to the newly-cut opening.
To finish it off, glue the 1x2 to the bottom of
the 2x4 as shown, screw the thumbscrew into place and you're set
for the clamp.
Note that it's really best to measure against
your exact pieces here, instead of following my directions. Hold
the 2x4 up against the center support and draw around it. Do the
same with one of the spacers behind the center support (or measure
if you can't get to it). My measurements should work for you, but
you'll get a better fit if you measure against the actual pieces.
This piece works best if it really *just* fits. That way, you can
raise the clamp and don't have to tighten the screw just to keep
it up for a minute while you raise the shelf, etc.