[Charlie B]

Without actually seeing what your trying to do I can't say for sure. But there are a couple of products that hold up well in a marine environment. They could be used to make either a filler or cover plate. One is King Starboard it is a teflon like plastic very durable and pretty easy to work with common wood working tools. It's the same or similar plastic that cutting boards are made of. Only downside is almost nothing sticks to it such as glue or paint. But that makes the finished product easy to keep clean. Another product is a PVC foam board sold as exterior trim boards. It is also easy to work with common wood working tools and can be glued with PVC cement and painted or even fiberglassed over. Goes by the trade names Azec or I think Veranda. Two different brands but similar product. Home Depot carries all these products so they are easy to get. If you have some woodworking skills and tools it should be easy to do. Good luck...Charlie