Great idea. Fits well, but I am using on a wooden boat with thicker transom. I will use them, but will attach a tube on the inside. If you had a version with a longer tube (about 2.5 inches-to penetrate 1 in ply and 1.5 frame) for most wooden boats, it would be perfect. the angle is perfect and prevents the drain to be off the bottom of the boat (in the air) when drilling ply perpendicular holes.
Angled Boat Drain Plug