I've been thinking about this a lot lately, and have done some looking around in the Reno racers (unlimited ) pics...
I like the tips on Dago Red...they have an up-turned sort of a mini winglet style.
http://www.pbase.com/wjpearce/dago03
Another view of the same type of tip.
http://www.pbase.com/wjpearce/image/26124640
#3 looks like it'd be the lowest drag?