Welcome Josh,
You have a good idea, and your instincts aren't too far off. I think you may actually benefit from adding frontal area. Here's why, the "break over" angle at the top of your front glass is awful. (Red Line Bottom Pic) If you notice, I don't think any modern vehicle does this anymore. So if you built up, it will give you a chance to soften this and make it great. This will also allow you to follow the template, (Blue Line) which affords the optimum aero drag reduction, to the back edge of your cap. If you build the extension onto that following the curve, you'll really have something that works.
You could leave the "Top Foil" (Green Thing on Top) out and minimize the frontal area by just building the extension (Green Thing on Back) carefully. (Bottom Pic) Notice it is difficult to know where to place the template, I put it where it felt about right to me. But I purposely curved the extension a little below it, but not much, I have seen this done on some modern vehicles and can't help but think going sub-template on a longer overall shaped vehicle tends to help minimize the wake area without creating too dramatic a breakover.
Your shape in the picture you provided as a "How's This" is far too sudden of a break at the peak, this I feel would not work too well, if at all.
Also take note of the up slope on the bottom of the extension, this was not an accident, you want to taper up in this "Rear Diffuser" area at about 10°. This will help a lot. Speaking of this, you may want to consider figuring out a way to attach a large rear extension by something other than just the hitch receiver, I think you'd be putting too much stress on the receiver and box. You may want to consider something which goes into the rear stake holes on your truck bed and goes back to help stabilize that load.
And Yes, you do want to taper in the sides of any rear extension, you don’t have to be very dramatic in how much it tapers in, about 8°-10°, but you for sure want something.
Anyway, here's my 2¢ in a picture to start the ball rolling. I bet a donut a bunch of folks don't like the idea of adding any height at all no matter the logic. Top Pic
The Ecomodder forum has funny limits on how big an image can be to save space I presume, anyway, here is a link to a bit bigger version of this same drawing.
Bigger Pic to Study