A guy here (owning a Inter-17) made himself a large rectangular box 3mtr x 1.8 mtr x 1.5 mrt and that fitted between his hulls. The boat lay on top of this cabin, layed up on the beams.

Indeed, he did not have standing room, but I think he could sit up straight. He had a door in the rear through which he could step into his cabin, even when the boat was on top.

The thing looked pretty nice as well. Not too big or anything.

Wouter