dpb writes:

Have application just begs for carriage bolt...have never seen punch to
create them--presume must exist? Something like a Greenlee but one
piece is 1-1/2 hollow square w/ 1/8" wall so couldn't get in even if
could deal with the thickness.

More than I care to file out by hand...easy alternatives, anybody??? I
suppose could just go on and use a truss head and hold the back side but
it's a stretch to reach all of 'em while trying to tighten...

It's reputedly possible to drill square holes. The drill is triangular
in cross section, tipped with three radially oriented chisel edges.
Each side of the (equilateral) triangle is equal in length to that of
a side of the desired square hole.

The workpiece is placed on two sets of rollers oriented at right angles
to each other so that the workpiece can oscillate. I suppose that the
drill speed has to be quite low and the workpiece quite light or
momentum of the workpiece would trash the drill.

Never seen it done, just saw some diagrams of how to do it. Do report
here if you figure out the details and give it a try.

--
Mike Spencer Nova Scotia, Canada