The basic design is the one that James Baldwin uses aboard Atom. A plywood flat top with hardwood supports. I felt that this offered the most utility in that I could set stuff on it and not have it roll off. Plus I sometimes use it as a big desk as I stand on the companionway ladder.
After lots of input from the group I went with 1 & 3/4 inch thick by 4 & 1/2 inch tall white oak supports with a 3/4 inch zero void marine plywood top. The white oak was purchased as raw 2 & 1/4 inch by 5 inch beams from a marine lumber yard. The rib on the trailing edge of the sea hood is 3/4 inch thick by 1.5 inches high and is stock oak that I picked up at a hardware store. The sea hood is held together with West System epoxy and close to 36 stainless steel screws.
The rib on the trailing edge really makes the sea hood work. Without it the top piece bends all over the place; with it the top is rock solid and it does a great job of deflecting dribbles of water away from the companionway. The finished sea hood weighs approximately 35 lbs.
One tricky part to building the sea hood is getting the curve of the support beams to exactly match the curve of the cabin top. I used a compass to transfer the curve of the cabin top to a thin piece of plywood which I then took back to my shop and built a big radius to match it. On my boat the cabin top curve was best approximated by using a 13 foot radius. I then took a router, attached it to the radius and cut the curve. ( See picture ) Note: the only curve you cut this way is the leading edge of the sea hood - the side support beams are cut on a table saw to an angle that matches the cabin top. Also note : always wear a respirator when doing this kind of work - (I was just posing for this picture ) the fine dust will really get to you if you don't. I use a full face unit made by 3M. I also used the router to cut a 1/2 inch radius on the top edges of the sea hood to make it safer as well as take away some of its angular look.
After I completed the sea hood I realized that through bolting the unit to the top of the boat would leave the plywood top venerable to water intrusion and eventual rot in much the same way that the core in a boat deck can rot around a fitting that is not sealed properly. To fix this I drilled out the mounting holes, ( in the 3/4" plywood top only!!! ), to 3/4" and filled with epoxy. This way the plywood would be completely protected. The joints were filleted with thickened epoxy, two coats of Interlux Brightside primer were laid down, and the unit was finished with three coats of white Interlux Brightside. The sea hood is attached to the cabin top with ten stainless steel 6" by 1/4 - 20 type bolts and is laid in a bed of 3M 4200 adhesive sealant.
