Congratulations ... nice to see folks giving back to the sport ... especially the young ones!
Just as a suggestion and not a criticism, whoever made the boxes should make the tops a little larger all the way around so there's an inch or so overhang which helps keep rain from leaking in.
Also, if they'll attach a small perch strip just below the entry hole, the ducks will have a much easier time landing there and climbing in. A 3" length of 1/2" quarter round or corner molding works great.
Edit: Opps, just now caught it. Looks like the boxes were made on the economy plan from all the same sized lumber. That's why there only an overhang on one side of the top. But there's no side access door for adding nesting materials, inspecting and cleaning out?
To access you just remove a few screws and the entire front comes off. I like your idea with the molding. I just might go back and add that.
Again ... not a criticism, trying to be helpful.
If you cut one of the side panels into portions of 1/5th - 4/5ths, nail or screw the lower portion onto the box permanently. Hinge the top side panel at the top so it will swing open just above the top of the nest inside. This way you can easily swing it open with NO TOOLS to inspect and replenish nesting materials if needed.
Use a simple wood strip screwed a little off-center as a bottom latch. Attach it to the bottom panel. That prevents the door from opening unless you rotate the latch out of the way to swing open. Rotate it back in place to hold shut.