I THINK sheet fiberglass is available, but making your own (wrap your original panel) allows you MUCH greater strength, and the ability to mold items into it (such as drink holders, storage bins, cubby holes, speaker boxes, or tool boxes), if you are so inclined.

HUMMER .25: No need to compensate.

1989 Dodge Raider: 3.0/5
1992 Chrysler LeBaron Convertible: 3.0/A
1994 GMC Safari: 4.3/A