If you taper properly before the flat cutoff a squared off back (Honda insight is the quintessential example) is fine because the real turbulence happens a few feet behind the vehicle. At speed a blunt (even if it's rounded) nose just isn't great no matter how you cut it but with careful attention to corners, shape transitions and whatnot you can mitigate that quite a lot (modern crossovers with their bulbous front bodywork make heavy use of this).
This is a subject with a million gotcha's that add up to a lot. Be weary of anyone peddling rule of thumb solutions.
Here's one data point, merely removing the rear spoiler an SUV has the same drag in reverse as it does going forwards: https://airshaper.com/blog/mercedes-eqc-drag-coefficient
If the same car had the fine details massaged for the reverse direction in a similar way to how they had already been done for going forwards (i.e. instead of the rear spoiler, putting a similar spoiler facing the opposite direction at the front of the car), the Cd would be significantly less in reverse.
I'd love to get it running again but cant find decent instructions for swapping the battery system out for a normal alternator and battery.
> most cars are more aerodynamic in reverse than they are going forwards
I tried to find more about it, but I could only find two sources. The first was a Mythbusters episode about a particular Porsche, and the second was a Top Gear episode about a different car. Can you point me to an article that substantiates this?