On my 21' Cal I have a "keel-board" that is approx. 6' long and sort of bottle shaped. It fits in the keel well, after the keel is lowered, and prevents virtually all water from entering the cockpit. Without dimensions I do not know how one would be constructed other than on the hard with the keel lowered. The wide part of the "bottle" fills the hole in the cockpit while the "neck" portion slides forward into the keel well that is under the cabin. Then there is a deck board that goes over the top to cover the well in the cockpit, but that, in and of itself, does nothing to keep water out. It just covers an 8" deep hole of the well.