I made a bottom plate for the board from solid carbon and 5mm 80g/m3 Diab hard foam.
Top plate is a cfk sandwhich from 5mm foam as well. See here:
Bottom: Daniel's 2nd build - #4 by sat_be
It’s a bit hard to see but if you scroll down you get the idea. I laminated the sandwhich with one layer on each side, glass on the inside, carbon on the outside. Then I glued it to the bottom of the board and laminated over. In your case I suggest you reinforce the area where the mast attaches with aditional carbon layers, or put some stripes of full carbon in the sandwhich of the bottom plate to distribute the forces.
Top plate: Daniel's 2nd build - #6 by WeightShift