The end pieces cover the ends of the trailer, and when the roof and hatch is done they provide a place where these pieces will meet. These end pieces have a beveled edge to match the curvature of the trailer and the pieces they will meet up against. Again it is attached with pocket screws and glue.
It is difficult to see here but there are shallow holes drilled with a spade bit on either side of the deck. When it is flipped these holes allow space for the exposed bolts on the trailer so the deck is able rest on the rails directly.
It is a good idea to make sure it fit on the trailer before gluing!