We haven't put a metal one on the back but I have installed a plastic one on our son's Scamp
13 and on our Scamp
You can see it under Modifications, Preparing a 1991 Scamp
Basiclly we remove the spare and mount a couple of shortened right angle L brackets to the bumper. We then mount a Walmart's plastic bin to brackets and remount the tire causing the back of the box to be trapped betwen the tire and rear of the trailer.
We use the box for water hoses, power cord, wooden leveling blocks and a small tool boxes plus some misc items. We put nothing too heavy in here.
It is not water tight but everythinf in here can get wet.
Additionally we have two 5"x5"x 4' fence posts mounted to the bottom of the bracket supporting the bumper box. We use one for a long sewer hose and the other for poles.
The box we bought was $18 about 3'x1.5 x 1 '. We removed the hinge and added two side latches. Glad to add more pictures if you're interested.
We've used this box for 6 years.