Today was mostly a breaking day. The printed half-shaft design melted itself apart after some high voltage testing for a couple of minutes, so it's going to need a redesign.
Looking closer, it appears that just the outboard end where it meets the axle is damaged; however, I'm going to update the design to be more uniformly bulkier.
After smelling some melting plastic while running the motor at 9V, it seemed like more cooling flow was going to be required to run the car at higher voltage. There seems to be plenty of area for flow in the front of the vehicle with a little bumper redesign to open things up.
The roof doesn't seem very promising. It's a relatively small opening and as far as I've seen it's only on the '05 Mustang.
The rear can be opened up significantly with a new design; however, the airflow is also maximally restricted by the mounting for the plastic clip that holds the body onto the chassis. Perhaps some more breaking is in order before it'll work to build up the max-airflow car.