Sounds like a little more than what I plan on doing... but similiar ideas. Here is a copy from what I posted earlier.
I hope to get the roof rack on tomorrow.
I will bolt it to the fiberglass roof and hope for the best. I will have a rubber 'gasket' made out of a plumbing coupler between the rack mounting foot and the fiber glass top. Then on the inside I will have a rubber washer, then a large fender washer, then a lock washer, then the nut.
I think that will stop the rack from wearing into the fiberglass over time because of vibrations. I will just have to keep an eye on it for a while to make sure it doesnt loosen up.