You want this to be as solid as you can make it, so attach some 8" wide strips of 1/2" to 3/4" plywood to the metal cross supports from front to back in about 4 different places. Use plywood, preferably hardwood plywood, because it doesn't warp. Use one of the Evercoat adhesives between the plywood strips and the metal supports, and then screw the strips to the metal. That's the base for your 1/4" luan plywood. Don't use crappy luan plywood, get the best quality you can find and put the best surface out which will make it easy to get a good bond between the PVC final layer and the luan for the top and trim contact adhesive. I did a hard headliner in a '47 Ford with a grid of hardwood plywood strips attached at the front, back , and sides. It was so strong I could actually hang from it with no problem. There are pictures of it in my project journal.
No one lives forever, the trick is creating something that will.