I have 25 visitors today but will post some pictures for you in the morning
I bought my box at Walmart. It was about 18 dollars.
Remove the cover from the Bumper box. In my case it required pulling a long pin out. I cut the pin holders off with an electric sabers saw after removing the pin.
I removed the spare tire and centered the box on it. Mark the ends of the box on the bumper. It may be necessary to notch the back of the box a little around the lower spare screw. When mounting the box be sure it is firmly against the trailer and on the bumper.
I began by mounting 3 18" right angle L brackets to the bumper. Before doing this I cut the short side so it was the length of the bumper. I may of cut the long side so it didn't stick out beyond the box the outside brackets are mounted so the bracket ends up under the front foot tab of the box. I get underneath the mounted brakes
with an electric drill and put a clearance hole thru the brakes
and came for screws. I don't the same thing in the rear corners of the box from the top sidevdrilling thru the bumper.
Since there is no center foot on the box you may need to add a 1/4" spacer between the bottom of the box and the center L bracket. I used painted plywood. Also any cut edge on the brackets was painted. Use lock washers on all screws. Make sure all brackets are flat with the top of the bumper.
The fence posts mount to the bottom of the L brackets before the box goes on permanently. I put four screws thru the bracket and the posts. To download this I held the posts up from the ground side and drilled thru the center of the u profile L bracket and put screws and nuts thru. The screws should be close to the posts corners to prevent interference with any thing you put in the fence posts. If you use one for a sewer hosedrill some drain holes in the bottom.
Please read this and don't hesitate to ask questions. I'll read it after our company leaves and attempt to improve it but it should get you started.