I have made good progress the last couple of weekends on this crazy project. Uprights for the box have been installed, and my home-built "trusses" that have a slight slope.
My home-built trusses are just 2x4s ripped down on an angle to allow a pitch for rain to not puddle up.
I also added in some inside corner trim. I thought water from rainy roads may penetrate from underneath at the edge, so I used a piece of cedar (from the old built-in shelves of the fireplace room) to trim out that potential path. Most of this wood I've used in this project is free: from shipping crates from big equipment at work, so I'm happy to re-use some of my old scraps from the house, too.