Don't really know how to deal with the fiberglass, but you should be able to find dozen sites about it. Just type like "fiberglass enclosure" in Yahoo, Google or something.
I did came across a page that was talking about sth. like that, but I'm not sure if it was fiberglass. The guy just took an aluminium foil and wrap it around the model of the enclosure inside of the trunk where he wanted the enclosure to be. Than he just sprayed this foil with many layers of the sprayed material (dunno what it was exactly) and waited couple of hours for each layer to dry. When the layers were thick enough he stopped spraying, brushed the thing around the edges to smoothen them and that was it.
P.s.: He used aluminium foil because it can be easily transformed to whatever shape you like and it keeps that shape. But more important is that this canned material doesnt attach to smooth surfaces so the foil can be easily removed when the proces is over.