Hi Steve.
Nice work. Here's another idea: You can also use Formica counter top material. It is not expensive, comes in a standard thickness of .035", will not warp and it will flex without permanently bending nor developing the wavy bumps fiberboard gets. Cut it so the back side (the rough side) faces outward. If you want to paint it, sand the texture on the back until it is smooth, prime it and spray it after the pieces have been cut. Or coat it with contact cement and cover with vinyl as you've done. The headliner (as you know) has compound/complex curves. You can use a blow drier to help Formica bend to make those curves (the ones at the front) look right. I would not use aluminum on the headliner.
You can also still get upholstery grade fiberboard (here:
https://www.perfectfit.com/15347/Chipboard-Car-Panelboard.html ) but the non-waterproof board must be sealed on both sides and at all edges well or it will shrink and warp.
