Here is a 20w solar panel and mount that I just made up. The panel itself is very strong, so I just added the bracket and mount. It turned out very stable and strong. I even leave it mounted when trailering. I've even sold a couple.
I am trying to fabricate a mount (finally) for my 24" x 20" 20w solar panel. I plan to use those fast mount rail clips to hold the mount to the stern rail. I found planks of press-board in a 1x2 at Home Depot, which have some coating on them, they are white, and are said to be water resistant. I presume this would not work as is, but would if I were to coat them with epoxy? I have also heard that home stores also carry a similar version made of vinyl, in like a 1x3? I presume this would work as is, as it would be similar to starboard.