Well okay, I found a giant TV box in the dumpster and spent about 2 hours cutting it up to fit the back of the car.
I just went to go try and fit it, and...I realized there is no way to strap it onto the car by myself, because the box is huge and I can't really hold it up while trying to secure the straps. Worse, I was counting on buckling the strap inside the car, which was never going to work.
I tossed that box tail into the trash
I'm staring at the back of the car trying to figure out another way to make a box cavity of significant length.