If the goal is to measure the aerodynamic drag force of a given shape, perhaps a different approach could work better for the DYI / smaller scale application. Depending on how much one cares about what is lost from a true wind tunnel.
At much slower wind speeds there is a measurable aerodynamic force on an object.
The more we minimize other forces we can get better resolution / accuracy.
A gauge to measure the back force of the object as it's aerodynamic drag pushes it away.
A large plate to support the vehicle ... line the plate and a section of floor with magnets to ( mag-Lev ) would remove rolling resistance losses from the equation... use guides around the edge of the plate to prevent the plate from moving out of the control area during testing or when the vehicle is driving into place... don't need much movement of the plate in the control are... just enough for the gauge to be able to measure the amount of wind pressure / force pushing the car.
Link
100 for $8+Shipping gives a combined ~267 Lbs repulsive force ... ~$160+Ship for magnetic repulsive force of ~5,340 Lbs should be good for many cars including the plate.
Of course there would be other costs as well... but it would be a less expensive way to measure the aerodynamic drag force for a given object at a given wind speed.
Doable in a normal sized garage... and should be able to get it reasonably accurate resolution at measuring the Aerodynamic Drag / Pushing force... it should be repeatable as well... to be able to retest after making small or temporary changes.
I know it's a crazy idea ... but aside from being crazy ... what to people think?