Hey Robot Slave, Been looking in on your build and really had nothing to add till today, I have come up with 2 different things, but it looks to me like you have the one idea covered.
1st I was going to say that your V1 drawing was too tapered, it's surprising how little a taper you need to be effective, but by moving the plan taper back on your rig, you fixed it! Good on ya!
Next, I want to suggest looking for a small FanTech fan for a vent fan, they're whisper quiet and very efficient. An FR125 will use only 20 watts of power, you could buy a cheap inverter to power it with 12v and just use 4" ducting to plumb it. It may take a little more ingenuity to figure out how & where to install it, but for about $110, you really have something that Sucks....er, I mean, Blows...uh, well you know what I mean. When I built houses a a few years back, I used these a lot. Insane quality for the price, the impellers look like aircraft parts.
FanTech Supplier Idea
FanTech Site
Look up NACA vents for providing airflow while the rig is moving down the highway, they are a low drag way to move air.
I would avoid putting anything in the slipstream if possible, it will really be counter productive to what you are trying to do here.
Yer #22 post looks like the way to go. My only question would be how you cover the front wheels, but you could save that for last.