To my knowledge that has never been done, and their are no approved installations. Probably the best option is something like the "Arctic Air" cooler,
www.arcticair.com. I know of at least one Comanche that has vents in the hat shelf at the rear of the cabin that are connected to an arctic air cooler in the baggage compartment.
Most pilots report that the arctic air is reasonably effective, and it has the advantage that you only need the weight of the cooler on board on hot days, where as you'll carry the 50 to 70 pounds of installed AC on every single flight whether you use it or not.
Jay