The shelf is made of 1/2" plywood with 3/4" pine on the front and underside. It is about 18" wide. I started by making a template of 1/8" masonite that just fit resting on the top of the hull. It is a matter of shaping the template to follow the curve of the hull and getting it flush with the wall of the boat that contains the anchor locker. Not difficult, just a lot of trial and error. Once I got the template, I cut the 1/2" plywood and trimmed out the front with 2 1/4 x 3/4" pine to form a 1" lip on the top face of the plywood, and about 3/4 " below the plywood. I added some additional 3/4" pine on the underside of the shelf, and about a 3" wide piece of pine to the bow side of the shelf that attaches to the anchor locker wall. You can see this piece above the anchor locker hatch. I screwed through this piece to attach the shelf to that anchor locker wall, above the hatch. The shelf is not attached to the hull, and just rests on the curve of the hull.
Hope this is clear. If not, please let me know any questions you may have. I find the shelf very useful in and of itself, and the rod storage works pretty well.