I made mine out of 1/4 inch birch ply Framed with 1X2 fir. Wrapped the outside in glass then gel coated it. Two coats of resin on the interior no glass. I put a 2 inch curve in the top for strength. I go 250 and no problems standing on it. Still Strong after 15 years. I backed the trailer into a tree without the boat on it and the 1 1/2 in galvanized support bar bent around the box. More than strong enough. I prefer and lipped slide on lid. You can get the lid out of the way. Get into the box then the boat is on the trailer and water tight.
About $150 in material and 20 hours of labor.