Take a look at the RAM mounting systems. They make it possible to easily remove the unit from the boat when not in use. I also like how simple they are to adjust for different light conditions. Here's a link to the rail mount adapter. I have one mounted on my V.V. and it's got a square frame, but it wasn't a problem. I think I have a pic in the photo gallery.

http://www.cabelas.com/cabelas/en/templates/product/standard-item.jhtml?id=0013692010094a&navAction=push&navCount=9&indexId=cat350011&podId=0013692&catalogCode=IE&parentId=cat350011&parentType=index&rid=&_DARGS=%2Fcabelas%2Fen%2Fcommon%2Fcatalog%2Fitem-link.jhtml_A&_DAV=MainCatcat21276&hasJS=true