The original builder of the Mangy did the same thing, cutting the fiberglass flush with the panel. I used the same carpet for the glare shield that I used on the rest of the interior. I cut it 4" long so that I could fold it over on itself for a 1-1/2 lip and 1/2" attached to the underside of the fiberglass.
This lip is flexible, yet rigid enough to hold position. It really serves no purpose but looks nicer than just a flush edge, IMO.