I put a piece of angle aluminum from the tailgate to the cab and used a speed square to measure the angle.
In the photo, the aluminum is sagging just a little, but accounting for that it looks like the angle is 12 or 13 degrees.
I was also wondering about doing one of those little squared-off windows - which car has that? The Prius?
Just with the plastic stretched over the back of the truck, it really limits rear visibility. It's nice to know who is behind me, especially since the drivers side mirror is not the original (a smaller sized replacement)
If I had just a small plexiglass window on the bottom edge of the aerocap, I would be able to see if someone is directly behind me or not. I am thinking a strip of plexiglass about 4 inches high?
I was also thinking that it would be nice if either the entire cap, or the middle section was easily removable. That would allow me to still load up the motorcycle or pickup landscaping materials, etc.