I made the bottom door out of .032 2024-T3; I bent an upper flange on the skin at the top and made a .020 right angle to smooth the inside edge. I secured the upper portion of the door with 6-32 truss head screws directed downward, 4-40 countersunk screws otherwise. There is enough room between the window frame and the door; this avoids a whole line of external fasteners and loads them in shear, as well. I made a cutout to mount an external vent. This nifty vent is from the RV accessory catalog, and should work well to direct air to the rear seat, since there will be no interior.
