You gotta understand that tripping up the air, particularly on the corners, is a dangerous thing to do since this is the area where vortices can and will form. And I mean huge, energy robbing, 18" diameter spinning tornadoes that require large amounts of energy to sustain. That 2" front top corner aero toe stub you have now could render the aero cap benefits useless. You might as well just make a traditional cap and take advantage of all the dry storage room. This is also why you want to ease the edges, sudden pressure changes start air spinning. I know, a vortex was the last thing you figured you'd need to be aware of avoiding, but they'll hose you over quick.
Yes, you need the curvey, it isn't the "Curvey" that makes it aero, it is the lack of an abrupt angle change that makes it good. You want the air to ease its way down from the center of your cab, and an abrupt change will not give you the same benefit. You could actually raise the whole back, without curves, to 10 inches and still get a good result and have more room.
To bend the board, get a piece of 1/4 inch plywood and make it in 3 layers, it will be bomb proof. Or you could cut slits in your solid lumber to get it to bend easier. It looks like you're using 1x4's for those corners, putting notches every 2 inches would do it I'd think at about 1/4 to 3/8 inches deep, and you won't lose much strength there.
Like this.
And I'd think 8 inches across most of the back is where you need to be. The back plate I illustrated is the shape you want to shoot for. You did a good job on your back plate! It's just not quite tall enough to be optimal.
If you need the front plate to be in the shape it is, then make it smaller so it tucks in behind the cab everywhere, this is a very critical area, things sticking up here, or too quick of a transition like you have now, will negatively affect the results in way greater proportion to their size. You'd be better off having it 2 inches lower across the whole top rather than 2 inches taller in the corners. If you put the wood in at the 45° angle, this should get it below the cab line.
Oh, and filling the gap between the cab and cap is not a big aero concern, you could leave a 2 inch space their all day and it would be insignificant, maybe a 2% difference in the improvement you get, so instead of going from 28 MPG to 32.2 MPG from you'd get 32.12 MPG.
Your current design might get you from 28 to 30 MPG where if you make the changes I'm talking about, you'll get to 31.5 or better MPG.
I'm not guessing here, what I tell you is based on a few years of contemplation over this.
Here is a paper upon which much of what I'm saying is based. Pay attention to pages 85, and 89-90. Real numbers, real results.
Drag Reduction of a Pickup using Add-On Devices
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