I've been playing with composite sandwich core boards for the better part of a year now. The one issue that has plagued me is the fin boxes. The common approach is to sink high density foam into the low density EPS core to help keep the fins from twisting.
The problem with this method is that it creates a weak spot. The high density foam doesn't flex with the same "ease" as the low density EPS surrounding it and so focuses "bending energy" along a very short span of the joint between the two different density foam.
Using pour foam, rather than gluing in high density foam risks melting the EPS along the joint as it gets HOT as it cures.
The only thing the high density foam is intended to do is prevent the fin boxes from twisting side-to-side under load.
One way to avoid all the complication is to sink the bottom of the box into the deck material. Both the bottom and the deck are high density foam. If the bottom of the fin box can be sunk into the deck skin, it provides two points of contact into the high density foam and I believe will prevent the twist off in the fin boxes.
Two pictures. The first showing the routing for the fin box on the bottom and the the deck side of the board showing that I am NOT a CNC machine and could have shortened up the depth by about 1/32" of an inch
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