Long time lurker, first time poster.
Robert Riley has a nice write-up on foam fiberglass one off work at
rqriley.com
Basically the way Tre stated he was going.
I talked to an aviation friend that stated the optimal angle of convergence depends on the velocity (and fluid density of course). The often quoted 14 deg angle is optimal for 110 MPH (IIRC) in air @ sea level. You may find when you increase your air speed that you will need to lower your angle to prevent seperation.
Basjoos put his exhaust and cabin air exit in the tail to help with filling the air void. Perhaps he can weigh in on any benefits. Motorcycle racers have found that the angle and location of the exhaust plume makes a fair difference in air drag. A change from a straight tip on the muffler to a rotated one can subtract a few hundredths off lap times.
Just my $0.02
Awesome job, keep up the good work.