OK, so I put all thoughts of electronics aside for a bit and I have the stern roller made ... I think ...
It is made of a piece of acrylic towel rail someone gave me "just in case". I picked this instead of metal as there should be no corrosion of any kind, it paints well and fiberglass resin sticks to it.
Now I need to cut the hole, then slot the tapered ends of the end bits so they fit over the hull sides. Some filling and sanding should see it soon done. As I read in another thread, I am treating each bit as a sererate project and when all the little projects are done, the big one should be as well.
Tom