The setup below is pretty versatile- the box holds lots of miscellaneii, and the rack can/has served in a variety of ways depending on who's along and where we're going.
The box began life as a crossbed pickup toolbox- I changed the hardware, set it on its side atop the bumper, and bolted it both to the bumper and the body of the trailer. This last because I wanted it to be as solid as possible, both for stability and to prevent it from carving a long gouge in the fiberglass due to vibration. (There's a long gasket between the box and the body).
Loading is definitely a balancing act...if there's a bike back there, compensatory rearranging occurs up front.
Another innovation you can sort of see in the picture is the access hatches (look like vents) at each end. I put those in all four corners, and since my trailer has the front bench, the upfront hatches open on a trailer-wide area perfect for long things like oars, fishing poles etc.