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For the size of this boat, a triangular sail isn't as efficient or powerful as a Leg-O-Mutton type. try Duckworks or check polytarp sails as a keyword, these kinds of sails put more working area up higher in the air while making it easy to duck under the boom. A Lateen type sail like those used on the Sunfish could also work.
The side area of the pontoon "amas" is large enough that you may not need to add a dagger/centerboard. You won;t ever be fast in this design because of the profile drag and wetted area of the pontoons, but that's okay. Use a surform sanding block to round off all the foam and wood edges. The lettering on the ink foam can be removed with alcohol, or sometimes the printing is on a removable clear shrink plastic layer. Nice simple design, could be stored as flat components.