Try using pipe insulation the foam tube type to fill the vacant space you were showing. Just cut the length you need and press it into the space. It will fill the space without warping it
Nice job, watchable video. How long have you had your Bronco? I'd think that the older the build of the roof the more "cured" the glue of the factory headliner would be. Do you see that a possibility?
Definitely. Especially as that glue thermocycles over the years. I've had mine for a couple of months. I suspect that even 5+ year old headliners aren't going to vary in tensile force to remove more than 25% or so.
Using a hair dryer (I’m not ever going to admit where I got the hair dryer, and tactically plan such use when SO is out of the house) to warm the glue might help remove the factory liner. A heat gun is much more SO approved, and made for garage use, but might be a little too hot.
You're changing the mass by adding insulation (square-root k/m), and changing k by pressing up on the headliner. Be careful about modifying too much during the warranty period, might null the warranty in some cases.
