I just looked at your project journal. I love your car, it looks a little like a sedan delivery now, and I love that look. You're going to be O.K. with the last section being 18" wide, you just need something to attach the headliner to above the rear window. Model "A"s had panels covering the whole rear end, and that's what I'd recommend you do. You will need some kind of framework to hold those panels as well, and that can be made of wood or metal.
Here's what an original "A" looks like in the rear. The panels have seams at the edges of the rear window, and another seam at the rear edge of the rear side windows. There is a separate narrow panel just above the rear window the width of the window.
No one lives forever, the trick is creating something that will.