I was fighting a water leak at the pass through window when I switched from a 2004 Dodge to a 2015 Ram where I had to raise the camper 3/4". Ended up putting a deflector across the middle of the cab over because I was able to use 4 existing holes in the sheet metal. It reduced the air pressure and changed the direction of the wind enough so the leak was stopped. The added benefit was it reduced the bug count at the top of the front wall under the cab over. It actually added a bit of wind noise in the cab and I didn't notice any change in fuel mileage.
If I was to add one to a fiberglass camper, I would make sure it is angled fore/aft and just use a strong double sided tape. One advantage to this is if you decide it doesn't do what you want, you can remove it without any damage occurring. Plus screw holes equals leak, especially in what will be a high pressure water driven area.