The boxes are about 8" deep (inboard-to- outboard). I built the first one at 18" long and then decided I want the other one a bit bigger so I made that one 20" long. The longer one got glass in on the port side since there will be few feet more spin sheet in there while gonig up wind.
The bottom of the box is parallel to the cockpit floor so water should drain aft. As you can see in the second pic, the top and bottom are not parallel---the bottom is slanted slightly inboard to facilitate draining to the rear inboard corner.
The opening is 5" x 11". I doubled up the edge with some 1"-wide scrap to provide a little added stiffness.
The other benefit I hope to get from these is added support under the cockpit sides where the crew will be sitting.
Jeff