It would be great to even just have something for the sides made out of the same fiberboard material as the headliner, so we can insulate behind it, and have a clean look. I would definitely buy that from a vendor. I wonder where I can get fiberboard that is the same white color as the headliner, so I can make something - any ideas?
Well, I'm thinking that Home Depot, Lowe's, et al have fiberboard, but the thicker stuff. Perhaps a place that specialized in plywoods, veneers and Formica?
Once the right material is found, it's be pretty easy to just spray it with Parchment interior dye, which most classic Mustang vendors sell if the Bronco places don't. The finishing touch would have to be the welting, which the factory headliner has around the bottom edges. I'm thinking an upholstery shop could get it, but you might have to buy some gigantic roll of like 100' or something.
But, bottom line: It'd be sweet if they were available ready-made. I'd pay something in the $150-200 range for a professionally done pair.