...Any thoughts?
Personally I wouldn't go with more than one fin unless I wanted to also have a wing, just more drag, although the drag from a 'somewhat' aero fin is not a lot. Then I'd only go with the one fin if I needed it to help with the center of pressure which looking at your cars profile it might need depending on anticipated speeds.
Is the added drag going to be less than possibly making the overall drag of the car lower? I would guess not, but that might only get answered in a wind tunnel or by Woody. The back of the car looks pretty clean to me so I don't see how the fin would help the overall Cd but that is purely a guess.
Keeping the angles that the sides go in and the top of the car tapers down to less than 7 degrees will always help as it is my understanding that angles of 7 degrees or less will help to keep turbulence down. That is what I've used for the whole back of my lakester.
Good luck,
Sumner