I finished the vent doors by priming the doubler frame and then riveted it to the fuselage on both sides. While I waited for primer -- and then hammered finish -- to dry on each side, I cut the hinge to 10" for the cowling attach point. I couldn't find a piece so I cut another 10" piece for the left side so I may have to order new hinge material later depending on how the rest of this goes. No big deal.
I drilled the hinge and shim piece to each side and then riveted them.
But the time all that was done, all the paint was done on the vent door material so riveted all that on and then assembled the vent doors with the called-out hardware.