This is a design for a simple 20ft trimaran based on a similar length tri by Dudley Dix yacht design, link can be found HERE

Study plans for this boat from Dudley Dix are only $40, (full set of plans is $200) the advantage being they would give a good idea of what strength timber is needed where etc. My point is that it should be easier to modify existing plans rather than start anew.

Space inside is going to be very limited is true, perhaps coach roof can pivot up to allow good sitting room of an evening, just an idea. As I have removed the side berths and the folding akas, overall weight would be much less than the 450kg specified. Thus reducing freeboard by an inch or 2 would seem prudent to decrease windage. Lateral resistance would be provided by leeboards so as to give a bit of space inside the cabin. I think the original designer did a good job with the hull lines, especially below the waterline. A design like this may appeal to someone on a budget that can live with a small cabin. It is at a guess half way between Kurt Hughes' 23ft small tri and the 18' microship small tri.