I've been thinking about your project as I look at it, and it is very good. I like how you kept it high to the tail lights. Also when you dive down at the end like that "sub template" I think it fills in the wake quicker and is a good thing.
If I had to wonder about something being sub optimal, it would be that it appears the sides do not taper in and the square side to top corners.
I think the sides not tapering makes the air stay at a "higher" pressure too far back, and when the side air does finally get to spill over and join the top air, there is a fair amount of pressure difference which could set the air into an energy robbing spin.
Do you plan on making something to sit on the tail gate which tapers in? If so, you may want to re-engineer the last 2 feet or so on the sides to taper in. If nothing is going to fill the gap there, you may want to consider removing the overhang so the air has a cleaner release and you reduce the potential for large pressure variations in close proximity to each other.
I know the square corners are a ease of build design compromise. I'm sure the overall reduction in them not being radiused is less than a 10% hit on what would be optimum. Keeping it high really helps that too, if you were more aggressive with your taper, then the square corners would tend be a larger concern.
Have you noticed how much more planted the truck feels at higher speeds? You've knocked the stuffing out of your lift coefficient so it should feel now like you're driving an arrow that just wants to go straight. Maybe the difference was more easily felt with my truck on icy roads, without the cap, it really skipped out on me once in a while with a cross wind, when the cap was on, it never did that.
Good on ya for gettin er dun!
Bigger Pic To Look At