you should make them with the same area of a nacra 5.2 or alittle larger. you can always cut them down later.
type daggerboard or something like naca foil in a search engine to get ideas for construction. it can be as easy as cuting them out of a sheet of aluminum and rounding down the corners.
a perfect fit in the trunk will minimze water shooting up the trunk which is drag. on the other hand, if sand gets in a trunk with a perfect fit, the board can get jammed.