I ran a primitive rally undertray in the past, but didn't like having all that metal under there holding the heat in. I had a noticeable increase in EGT temps. So I sold it shortly after putting it on.
I'd like to get something that's really low to the ground to keep the air out from under the car w/o having to go to a full undertray. Something stiff to hold out the air, but flexible enough to not get trashed by the curbs at the track (or cones for that matter). I'm really leaning towards the ghetto lawn stripping from home depot, but I'm not sure if it's stiff enough not to just become a big flapping piece of plastic over 100 mph.
This stuff: