1/2 inch is overkill. Like a boat the strength comes from the frame not the skin. I used 1/4 birch with 1 layer of glass and it won't even twist it you pick it up by the corner. For a frame I used 1X2 fir that was screwed and glued at the corners and edges. I ran 4 support ribs across the inside of the bottom. This made it easy to seam the ply togeather to get the length and it helps with air circulation if you have to put away wet gear. Make the top curved for strength and to keep water from pooling on the top. Easy to do. Cut 5 ribs(each end and 3 in the middle) the desired curve out of 1X4, then just screw(start at one edge and work you way across) the 1/4 in ply to the ribs with some brass flat head wood screws, use good wood glue to help the bond. I finished the edges with 1X2 fir. gave it a good coat of resin then a wrapped the outside with a layer of glass. Hand painted with Gel. In 15 years I have had very little maintenance.
I prefer a lift off lid for easy access and water tight. You only need 2" clearance between the boat and the box to just slid the lid off when the boat is on the trailer.