This is all such awesome information, thanks everybody for contributing.
I've been thinking about laying up a flattrack-style seat for my XR, but I'm too buried in projects to take the time for the whole plug-mold-part thing. Can I get a decent 20-footer by laying the seat right over a foam core? For a seat like that which is going to have some compound curves should I try to use cloth or is mat okay? It seems like mat would be easier to conform to the shape. Then smooth the outside with Bondo, sand, and paint?