FWIW, I'll attempt to describe the solution I came up with. It's similar I believe to what Boomer described:
I used two 4-foot long pieces of 2" wide by 1/8" thick steel strips (HD hardware section) as "hangers" to support the spar by the flange as I hung it off the edge of my table.
Picture these two steel strips laying on my table, with the ends protruding past the edge by a couple of inches. This allowed me to hang the spar by the underside of the flange that I was about to countersink, with the rest of the spar (web, bottom flange, mounting brackets, etc.) dangling below the edge of the work surface.
The steel strips were heavy enough in their own right to support the weight of the spar without clamping, but for good measure, and because I would be pressing down with the drill, I clamped and added additional weight to hold them in place.
I prepared the ends of the steel strips first by rounding the 2" edge off and deburring it a bit so it wouldn't scratch the spar. I also wrapped a thin layer of masking tape to further protect the spar.
I drilled a 1/4" hole in one of the steel strips, right near the end, to allow the pilot of the countersink cutter to protrude through the underside of the spar flange as I cut without hitting the steel.
I used thin strip of scrap aluminum (also from HD) double-sided taped to the same piece of steel that I put the relief hole in, and aligned with the edge of the spar flange when it was hung in place for drilling, to help support the part of the C-Sink cage that hung over the spar flange so the cage wouldn't tilt.
With this setup I was able to align each hole in the spar flange with the hole in the underlying steel, countersink the hole, then slide the spar a tad to align with the next hole. I used a small spring clamp mostly as an extra hand to hold stuff still while I drilled. I had my wife stationed by the other steel hanger, which was just to support the end of the spar that I wasn't working on, in order to keep it from accidentally falling off as I slid the spar from one hole to the next.
For the front spar, I had to adjust the strips of aluminum supporting the cage (add another layer) when I did the holes that also include spar caps under the spar flanges. And for the rear spar, the steel strips needed to protrude past the edge of the table enough to allow the elevator hinges to not hit the table itself.
All in all, this method worked great. Not saying it's the only way or the best way, but it worked ok for my purposes, held everything stable, and allowed me to get into a rhythm and have consistent results.
Last edited by missile29 : 11-24-2021 at 12:15 PM.