Double check with others for precise angles and whatnot, but I'm thinking this is about what you'd want to do..
Angle your "horizontal" flat piece up at the back, from bottom of car seats to about 2/3rds up the back of the tire. Approx 12*?
Then for the bike, 'channel' out about 1' wide, and build vertical walls down. The idea is to flow the air smoothly under the back end, while channeling air around the tire that sticks out.