A few thoughts. I have been trying to put them together into a complete concept but I am not there yet.

Thought #1: You could use the center wall (running from front to back) as a structural element. It could provide the structure for a skin to tension against.

Thought #2: You could extend the front of the enclosed volume out over the tounge and taper it. This could give you the curved shaped (or at least part of it) you are looking for to make skinning work. The extra volume could be for gear storage. the tapered shape would help with aerodynamics and provide clearance for sharp turns.

Thought #3: You could provide access at the the back. this would preserve most of the skinned surface. Rollers would allow kayaks to be loaded form the rear.

Thought #4: Cockpit or hatch style openings could give access for tying down kayaks or reaching forward storage areas.

Thought #5: Maybe you have a flat roof or maybe it is rounded (like an inverted hull).

Like I said, that is not a complete design, but maybe there is a way to get to one. I wouldn't give up just yet.

Steve