There are many boat plans available for home building, I have seen two that have been produced by some interesting naval architect and that might give you some ideas:

Kohler has a a 14' cat

http://www.ikarus342000.com/SC435page.htm
It might not look that good but it gives you a idea of the project.

He also has a twin rig 15' cat

http://www.ikarus342000.com/DUO425page.htm

The other architect is Richard Woods and he has a few plans for beach catamarans. The easiest plan would probably be the Pixie:


http://www.sailingcatamarans.com/pixie.htm

He also have this plan for a very interesting sailboat, the Stealth:

http://www.sailingcatamarans.com/stealth.htm