I went through and did all the countersinking on one of the spars, and all the angle pieces. For the angle, I had the drill press set up and it worked quite well. I have noticed that when countersinking this thickness, there is very little support for the pilot on the cutter. This causes holes to occasionally wander a bit, enlarging the hole. The drill press was able to fix that.

On the spars, however, the drill press doesnt work as well. The spars have an angle to the flange (not perfectly 90 degrees) so the setup becomes quite cumbersome. I found if I create a scrap piece with a #40 hole in it, this was enough to support the pilot of the cutter and create a nice hole. The off cuts on the angle bracket for h-stab attachment were perfect for the job.