How are you going to get from a foam core to a finished daggerboard that won't break? By the time you glass it w/ many layers of carbon or whatever the shape won't be the same as the intended foil.
A wood core milled on a cnc machine would need minimal glassing. you can make foil templates to check your curves, and shape a wood blade by hand too.
the blade should be nearly the same shpe as the opening of the trunk under water or excessive water will shoot in the trunck causing lots o'drag.
on the other hand, anything will work, and a sheet of aluminum w/rounded corners will act as a daggerboard. no kidding they exist and do work.
hope this helps.