This is the same box trailer on which I just replaced tires. The cap on the pop-up vent had degraded over time (12+ years?) and had been taped over multiple times. Initially the tape was to cover a crack, eventually it was the only thing holding things together.
The replacement cap is made by Camco. Their website has a good series of illustrations to help identify what model cap you need for your given trailer vent (there are quite a few variations of hinge for 14"x14" vents). I went with the "unbreakable" polycarbonate version & ordered through Amazon for about $24, delivered.
The pics are post-removal of the old cap, but the steps involved are all there.
The old & new cap after initial placement.
This style of hinge has a kind of "P" shaped channel on base & "U" shaped on on the cap, one nesting in the other. Tabs are bent in at the ends keep things lined up. With everything else removed, unbend a tab on the old cap & slide it off. Bend one tab on the new one & slide it on.
Close the cap on the opening & go inside
Insert the crank mechanism arm in the cap slot and secure the mechanism with two screws.
The screen goes back on, secured with two screws.
With the screen on, put the crank arm back on & secure with one screw.
The trim ring goes on, secured with four screws.
The last step is going up on the opposite side of the trailer to bend the other end tab to keep the cap secure.
The old cap material was pretty thin & got brittle over time.
It was definitely past time to get this done & turned out to be much simpler that I had originally worried it was going to be.