The spray on bedliner stuff works if you have a solid surface under it, but it is not going to give you a custom interior. This is something you would use if you have a really hard shape to cover. Here's a thread of the bedliner being used on a fiberglass headliner:CLICK HERE
I checked this link, and it doesn't work. Search for fiberglass headliner, and look for a thread by Centerline entitled "fiberglass headliner is done".