We've tried front and rear undertrays on a GTSt track car, the front on a couple of attemps certainly had good effect, but it needs to run atleast to the start of the gearbox to really work. The second time we ran a leading edge of about 3" overhang out the front under a GTR lip, it was constructed from marine ply. The first was all mounted inside the lip and was a zinc sheet shaped to suit.
For the rear we basically looked at the R34 V-Spec unit but made it longer and lower with the rear being higher than the forward section. It was also constructed of marine ply and it had four veins running straight. We didn't really gather any data on the rear tray as the car got cleaned up from behind by another car first day out... the longer front lip was removed in a gravel trap at the next meeting when the car went as close to rolling as physics allow... so we gave up!
We didn't see any overheating problems with what we trailed, but if too much was enclosed without consideration for extracting air I would expect this to become a problem.
I'll try and find some photos... looked trick and got plenty of attention from the punters, so who cares if it worked or not!