Seadog makes some fiberglass
reinforced nylon deck fills. The material looks similar to Forespar Marlon. We've been selling them for several years and none have come back. I've never used one but they look ok. I would be concerned that fuel
nozzles could damage the threads if you weren't careful. They don't address the UV issue on their website but I would choose the black ones over the white ones because the black pigment should give some UV protection. I like that they have different colored caps available to color code what they are used for. They also say what they are for. Here is a link to the Seadog website: Sea-Dog : Quality Marine, Industrial and Rigging Hardware
I don't have them up on my website yet but we do stock them.
Bronze or stainless should last the life of the boat, these probably won't.
Peggy, when are you going to visit Miami