I believe there is a better way than a wedge.
On the wind tunnel aspect, I've looked into renting time in a tunnel. The key is being able to Use the information. Don't forget, because the floor isn't moving, the air under the kart is not correct. Because the tires aren't rolling, the air around them isn't correct. You are wanting to gauge the aero qualities at relatively slow velocities and you already have several obstacles to getting realistic readings and then you have to figure out how your readings relate to the real world. Guys go to school for years to be able to figure all that out.
Get this, Ferrari's F1 team's issues this year we're attributed to not being able to figure out the relationship of their tunnel to real world. They are now using the old Toyota F1 tunnel. That's how important being able to correlate findings can be.
It is easier (AND CHEAPER) to rent an asphalt track for a day or 7 and just test, test, test.....
Mike