Thank you HMSBeagle, I will do the testing and find some flexible XPS. I will need to cut the headliner along two lines because one edge of the loose panel will fold down giving the access needed and the other is at the outside wall. The headliner is boat headliner, stretches both directions, is available at West Marine and will allow me to make hemmed trim that will get hot glued to hide the cut line in the headliner.
This will be a great learning experience and may find some areas that will benefit from some spray foam between panels as they feel like there are some small gaps where the loose panel doesn't fit snug to the one next to it. Spray foam itself has an adhesive quality as long as you use it sparingly so as to not distort anything during it's expansion time frame.
