why not just underclock the chip... IMO I would not cut any holes in the outer panels. sounds like trouble. and if I were buying a car and saw you cut a hole, I dont care what the reason I wouldnt pay 2cents for the car.
Id definitly investigate a better way than that. under the car could be extremely hot anyway, hotter than inside, black pavement and the heat of the transmission and motor all under there. naw, wouldnt do that at all.
Id try underclocking it. chances are you could run pretty safe at 600mhz or something, and that should keep the chip considerably cooler.