Had one too along time ago , had two straps holding it place, the only thing I can think of is going to a home A/c repair shop that makes ducts for the a/c units inside , they have the stiff board type of fiberglass you may be looking for , or run to a wrecking yard and look for a town car that has the same and cut it down , I do know the Bobcats were very quite on the inside because they have extra insulation installed , nice

Know what you mean , the early pinto had the straps , later ones had the ones you got, and I think the newer cars do also , like to find a bobcat wagon , those rode so nice