I am assuming that there was bench seats in the boat originally therefore, there are probably some mounting holes in the hull for the brackets (may be filled with blank rivets?). That is where I would reinstall the new brackets, using angle like you said. then you simply measure the distance between the hull on top of the bracket. Width is your call maybe 10 or 12 inches. I would not use composite material since it has virtually no structural rigidity or stiffness to it and it will sag like crazy. I would either use a solid board or 3/4" plywood. The wood will need to be completely sealed of course.