I attached mine with 2 SS 1/4x20 bolts and 3M 5200. The bolt holes are countersunk. Ran a bead of 5200 around the back and the bolt holes, also a little in the front holes. Put it in place and tightened the bolts and let the 5200 dry & cure, takes several days depending on the weather.
No leaks and only 2 holes below the water line