I finally got another chunk of tubing to finish the job courtesy of a friend of mine, Mark Barker. I want the front of the support even with the front of the aft spar carry through. So I made two wood blocks with a cross piece near the end, screwed on at a 90 degree angle to the wood. I also cut the wood to be exactly three inches high. With the blocks clamped in place and the wood cross piece up against the spar carry through, I was able to fit the aft mount in place. It worked really slick. The 3 inches made the top of the tube exactly 3 1/2 inches above the carry through and perpendicular to the top of the fuselage. I fit the two standoffs and then welded it all in place. The front of the tube is flush with the front of the spar so I can make a nice trim plate to fit...eventually!
