20 mm thick isn't very thick. This is why shinn puts the carbon plate on the bottom for the foils But here is what I am thinking #1 I would make a carbon stringer for it . if not 3-4 of them with abs rails #2 4oz 2x2 twill, HM12oz uni,HM 8oz biax +-45 (so it crosses the board at 45degree,2mm soric ,...