There are some good stock small trimaran designs around. I think both Chris White and John Marples offer them.
If you wanted to make something from recycled beachcat hulls and something (??) for the main hull, what are the major aspects you need to cover to ensure you get something sailable when finished? My short list would be:
1. Is there enough righting moment available for the rig?
2. Is there enough lateral aera for it to go to windward? (at least 12% of sail area)
3. Is there enough rudder to provide seerage? (at least 4% of sail area)
4. Does the center of effort of sails (centroid of both sails together) lead the center of lateral resistance (cenrtoid of rudder/boards together) by at least 8%? This ensures balance under sail.
5. Will the boat float on her intended lines? Need to check fore/aft bouyancy distribution.
6. It the platform strong and stiff enough to resist normal sailing loads? Look at what works for other designers and follow their lead.

This is my list of the top six. Did I forget anything?

Kevin