Bobhenry, thanks for all the great ideas! I've got a trailer, but it's not licensed yet so I've got to get it cleaned up and deal with DMV or whatever.
So, here's my thoughts on the build...
I'm hoping that the gap between the trailer and the canopy is solved with some sealant but also by the lip on the canopy hanging over my base wall.
For the door in the back, I am hoping to get a canopy like this:
That's why I'm hoping this will potentially be such a good deal. A used canopy provides a roof, windows, a man door, and a hatch, for the price of one door on ebay.
For the wind bracing, i like where you are going with the diagonal braces, but do you think the sheathing would provide that lateral support?
Great idea on the welded angle iron to stiffen up the rear corners. I have been worried about them being floppy, and the opening on the canopy means that whole wall is unsupported from slowly splitting apart sideways.