The lid on my 416 was built by the PO. He did a good enough job that I have not redone it. I have supplemented the nails he used with screws, caulked and painted it, and so on.
He built a 2x3 frame that sits inside the tub with one crossmember in the center, parallel to the axle. It is skinned with 1/2" exterior plywood. Since the trailer is greater than 4' wide there is a seam down the center - front to back. He placed a 1x6 in the center with plywood on each side. The 1x had some rot due to water intrusion so I replaced it with a strip of exterior plywood. The two seams down the centerline are sealed with another strip of plywood that is slightly wider, caulked and screwed.
You lose a little interior height right around the edges. Has not been an issue. You can load flush to the top of the side rails everywhere else. If the load requires and permits, the lid can stand proud of the trailer since it gets lashed down anyway.
Not having hinges or struts is a huge convenience. You simply lift the top from the side you need to access from, and prop it up with one or two of the handy 1 x 6 planks I keep inside. They are cut to a length such that they can be used as legs when using the lid as a table. Just slide the top to whatever side of the trailer you want the table to be at, let it sit on the lip, and prop up the distal end with the legs. A couple of ratchet straps top pull the lid back towards the trailer, and it is sturdy as heck.
The lid is heavy but manageable. In practice you never have to lift it. You either are just lifting one end, or sliding it along the top of the 416. If you need to move it, just walk it catty-corner.
It's waterproof but a driving rain will sneak a little past the edges. I carry a small tarp to throw over the lid for when it needs to be completely dry underneath and the weather is bad.
It is crudely simple and terrifically flexible. When I got the trailer, I thought this lid would be the first thing to go. Instead, it is about the handiest feature.