Hey, Lem~
Stroll down the insulation aisle at Home Despot or Lowe's and see what strikes your fancy.

While you're there, consider what it would take to use a half sheet of Luann to make a "halo" support you could glue or rivet the headliner to. "Halo" meaning not the whole area of the roof, but the flattest area of it, in which the thin plywood would offer some straightening-out support of the headliner. Another approach would be to glue such a support onto the headliner while on a flat surface: use the Locktite PL concrete caulking, and it will outlast the 3rd generation of you grandchildren. Weigh it down with bricks or your mother-in-law, and let it set 24 hours.

A simple suggestion from a simple mind.

Since you have that open, NOW is the time to think about extra map lights and stuff. Remember: the latex trailer-hitch testicles are illegal in Virginia, so if you hang those from the headliner, be sure to have some discrete covering for them should you drive here. A jock strap comes to mind