Here's what I did (dinghy say on aft deck roof): I took two pieces of leftover R10 foam and whittled away at them until they fit the shape of the hull, one fore and one aft.  I then used those as templates to cut out appropriate sized pieces of big box store lumber.  Coated in epoxy. Through bolted (drill-fill-drill with butyl tape) onto roof.  "Caulked" where the wood met fiberglass with thickened epoxy, then painted the brackets.

   Sorry, I mixing up two differents projects.  Lag screws from the underside of the cabin roof, through the fiberglass into bottom of the wood cradle brackets.  Not thru bolts.