In a process I will describe in a separate posting, I made a poster board template of the top skin and transferred the holes to my .020" sheet aluminum. It worked like a charn and every single hole lined up perfectly. Then I clamped the side skin over the top skin edge with aluminum extrusion and a 1x2 piece of lumber. That worked really well. By the way, my baggage back bulkhead is exactly 84 degrees. I took my sweet time -- six hours -- but the end result was outstanding.
Holding the top & side skins together with 3/4x3/4 extrusion and wood