Here's what I came up with:
http://www.hotboatpics.com/pics/data...rames1-med.jpg http://www.hotboatpics.com/pics/data...frames-med.jpg
I used 1/2" exterior grade plywood. All joints were glued with Nail It adhesive and stapled(industrial staple gunned) together. To make the bends on the foward seat backs, I cut 3/16" deep vertical slots and bent them around the guide pieces on the bottom. I also reinforced the foward seat backs with fiberglass. I used a router to radius all the edges. After they were done, just before upholstery, I sealed them with Thompson's water seal. I put four applications on them.
The original seat were the same material, nail gunned together(no glue) also. They held up for almost 25 years.
Here's what they look like now:
http://www.hotboatpics.com/pics/data...terior-med.jpg
http://www.hotboatpics.com/pics/data...erior1-med.jpg
They seem to be holding up well.