If you are having difficulty with the headliner sticking, you can do it another way.
Te Pantera is a lot like the headliner of a pickup truck.
You can use a time proven method of cutting a hard board and holding it in place with the edge moldings.
IF you need to go that route don't use cardboard. Get a door skin and use that. You will probably need to order one from your lumber yard but you can get one. \
They will hold up and are nice and thin. about 1/32".
The amount of headroom you will loose is negligible.
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