Hi Will,
Very thick boards become “real” displacement hulls since they ride so high in water all the time, not just at planing speed. To control this, rail, rocker, and bottom changes are made relative to the board length. These changes are generally less rocker than normal boards, and often more turned down rails. Deeper bottom contours will also be found. Check out any good SUP or tandem board designs and you’ll see various takes on this.