Sorry for the slow response.
I don’t have the dimensions on hand for the box, but we made it out of 1/2″ plywood (not the fancy stuff, just home depot plywood) to fit in the space. It was kind of fun to design because there was the angle of declination for the solar panel as well as the angle of the sides which we attempted to have mirror the angle of the trailer tongue underneath it. We cut it out very carefully with all the angles and then used wood glue and pin nails to put it together. I think we used old door hinges for the hinge.
Solar panel is 100W. Its overkill I think, but it was only $30 more than doing 50W, and I like the comfort of knowing I’ll never lose power to the fan in the summer. Also I can charge laptops and stuff if I need to.
As far as the axle, we spent a lot of time thinking about it. We considered moving the axle, but in the end decided to leave it where it was. We did some measurements to make sure we would still be able to turn the car sharply and there were no problems. We figured that the trailer is operating as Harbor Freight designed it to. So far there have been no problems. It pulls really nice and is super easy to back up.