I have been pondering the same question, altho I still have a ways to go before the headliner installation. contact cement is fairly permanent, or at least somewhat destructive to the foam if you try to rip it off. what about trying Scotch spray adhesive, which I think is a weaker form of contact cement. people use it to temporarily glue patterns to wood for cutting out pattern pieces. even though the spray version is “weaker”, the large surface area of the foam pieces relative to their weight might allow the pieces to stay in place until you want to remove them. my TD is upside down right now while I work on the bottom, but when I flip it later in the week I’ll do a test with a couple of headliner sections. I really like the idea of a reversible installation.