I think putting it and the Porsche Type 64 in a modern windtunnel gives a basis for comparison (fudge factor) of the facilities and protocols.
This is a work in progress. I got to thinking if you cut the CD from 0.40 to 0.20 you could double the area and have the same drag. If the B-pillar is moved out 3 inches then the body engulfs the rear wheels. Add 3 inches to the top and you are adding about 3 sq ft to 18 or 19; maybe 15%. Essentially, it would be a geodesic template with the Beetle decklid and fenders grafted on.
Welding rod covered with doped fabric?