I made up some wedges to fill the gap between the lower flange of the joint plate and the canopy frame. To get a perfect match, I first made sure these flanges were perfectly square. Then, with the joint plate clamped in place, I pushed in a scrap of 1/8 thick aluminum and measured how far from the aft edge of this scrap to the joint between the canopy frame and the side rail. Next, I marked that same distance on one side of a square (from the square corner). To setup my disc sander for the correct angle, I used the square (vertex held against the disc) and that piece of 1/8" aluminum in between the disc and the square with the edge of that aluminum at the mark on the square. Then, holding that square down to the table, I was able to loosen the miter gauge and adjust it to the other leg of the square. With the miter gauge tightened down, I sanded a block of wood down. Now this block of wood has the correct angle. I then resquared the miter gauge and used this block of wood as a correctly-angled backup while I sanded down a piece of aluminum to have the correct angle.

A view from the outside.