I have replaced a few, they are shaped like a cross with the horizonal piece shaped the same as where the hull and deck meet. The fitting is glassed in position. I used a 6 inch hole saw (in reverse) on the opposite hull side to gain access. Save the cutout, Ground away the glass removed, installed the new part, held it in place using vise grips on the outside of the hull, glassed in. Repaired the hole using the cutout and a backing.