Now I understand the idea. Flat strap loaded in compression like that isn't very strong. The internal frame idea would be better. Basically you're building support columns inside of the shell to support where the rack's 'feet' land on the top of the shell.
Those columns and arches could be fiberglass and physically be part of the shell itself. Rough up the fiberglass where the supports want to be and use cardboard to form the shape of the support columns or arches and stringers, and tape it all in place. "Paint" the cardboard with resin. Use Bondo to provide a fillet in the inside corners so that the glass cloth will form into them better. Bondo is basically polyester resin and talc powder, so it will bond to the polyester resin likely used in the mfg of the shell.
Once all of that is in place start laminating fiberglass over all of cardboard. It's whole purpose is to define the shape, but leaving it inside of the fiberglass won't hurt anything. The size and shape of the arches/columns and stringer determines the strength. For compression the larger the perimeter distance the better. For the roof bending resistant stringers the larger the vertical dimension is the better. For ease of use I think that I'd use the width of the mounting flange for the width of the columns and arches.
Have a look at this page:
http://www.fibreglast.com/
EDIT: Something that looks like this: