Well, ideally you want to tape up everything that is not smooth, i.e.
Tape up all the vents on the front bumper, kidney grills included
WARNING! This WILL cause your car to overheat! But it may be good for 1 lap
Then you want to tape up all the panel gaps (i.e. bonnet to fenders, around doors, etc.)
Additionally, you should also tape up the windows (i.e. stick some tape over the rubber/glass part)
IMHO, you're only going to get like 0.2s max out of this