Shouldn't be hard to create something out of plastic. Make the top surface out of a single piece of plastic, above the level of the roof and overlap the roof for an inch or so. Either use a thin piece of plastic to hold the plastic above the roof level, or just build up the fiberglass a bit. Seal with the putty stuff. Leaks
should not be an issue because water doesn't flow up hill except maybe with capillary action. If you try and do anything flush or sunk down like a car sun roof it will leak. Have fun. I thought about doing one for a while, but decided a vent was a better idea because the sun is brutal in Colorado.