Fiberglass is easy to work with and a fairly inexpensive product to purchase. Depending on what is wrong with the cap, it might be easier to rebuild the existing part instead of replacing it. Many time the original part can be made again serviceable with additional layers of glass and resin. The original part becomes a male mold and supports the new product until the resin hardens. The new area is then sanded smooth and surfaced, and when finished, can look better than the factory OEM part.

Fiberglass is damaged by UV rays. Its not if it will be damaged, but when. This will depend on how it was constructed. The better roofs have a gell coating, some do not. Any fiberglass roof may be improved by adding a coating. A good quality paint,(marine and aircraft are the best), or an elastomeric coating, will go a long way to protecting the roof from the sunlight. Do not use any asphalt based mobile roof coatings.