Getting back to the corrugated plastic sheeting. I deal with that stuff every day. It is not easy to work with at all. We die cut it and make different packaging parts. As far as forming the plastic to the shape needed for the headliner........If you had a heat gun and a premade form that you knew had the proper curves, then you may get lucky. I wouldn't use anything heaver that 5 mm as this stuff is tough. It has memory and unless you heat bend to change the actual makeup of the plastic, it wants to return to its original shape. As far as gluing to it.........Cold glue will not work, (elmers type)....You would have to use something like a 3M spray adhesive for good sticking. The surface is slick. If you want to try your luck as working with a piece without buying some, just go out and find an old election sign. Most of them in plastic is in 4mm so it would give you an idea of what you are up against............Good Luck.........
