I once saw a set that were custom made for a car, and they barely stuck out at all. They mounted THROUGH the door, and had a flat lense and a mirror and were able to give you the same view of the road that a normal mirror would give. They were super cool, and I haven't been able to find the web page again...
Maybe someone else can find it! I would like to see them again. I don't know if you could purchase them, but I would guess they would be expensive, but super cool.