I've been toying with the idea of building a sit-on-top/surfski SOF.
As step one of the process, I built a SOF with a thin plywood skin from the front of the cockpit to aft of a hatch behind the cockpit. The ply made mounting the hatch and cockpit rim simple. It also provides a rigid surface for supporting the boat on roof racks. I stapled the skin in place around the area to be covered by the ply - wrapped around the inside of the gunwales and the frames - then mounted the ply with screws and caulking after painting the skin.
The challenge appears to be getting a self-draining cockpit attached to the frame. It should be doable using 1/4 inch (or thinner) plywood stitched together and glassed. Could you lay up a glass cockpit using a sit-on-top kayak as a female mold? The drains would have to be attached at the low point of the cockpit and run through flex tubing to through-hulls. Could you glue small plywood pads to the skin to accept the through-hulls? Would the weight of the cockpit make the boat unacceptably heavy? Wound the height of the cockpit required to make it self draining and the height required to provide clearance between the drains and the skin make the boat unacceptably tender?
One of Jeff's longer designs should make a good starting point for the hull. Any ideas, suggestions, or experience to support taking this to the next step?
Go to kayakfoundry and try to design you're own..it's free....