I epoxied and fiberglassed the seat mounting frames in place last night. The frames are made of 1/2" plywood fiberglassed on both sides. Several sets of stainless steel nuts are embedded in the frames to allow for a variety of seat heights and angles. The seats will be held to the frames using aluminum angle brackets and thumb screws.
Also, the trailer is mostly assembled... I still need to put on the plywood decking, mount the bunks, add the tongue extension, and install the wiring and lights: