Here is the situation. I have an old house with window units. The unit in question is actually in a dedicated window (i.e., it is not regular window, but simply a rectangular cut in the wall in which the window unit takes up all the area.) The outside of this unit is currently the carport, which will be enclosed into a garage. I also plan to put a window unit for the garage itself to use during summer nights (with its metal only roof, the garage is going to be far too hot to keep cool during the day, and I can easily do what I want to do there at night.)

Anyway, obviously I need to somehow have the current window unit in question vent to the outside. The top of this unit is only about 1-1/2' from the carport/garage roof, and it is in a corner, so there are 2 walls already there. It seems that I should be able to build a small enclosure of OSB or metal duct (or maybe flexible duct?) for the base and the 2 walls up to the roof, and then cut a rectangle in the roof, and then put some type of duct out to keep the rain from coming in. I suppose that it could be a good idea to put some type of fan there to get enough ventilation as well. Then there is the question of how much headroom there should be in front of the outside of that unit.

I probably should take a picture or make a drawing, but I hope I have explained it properly. Of course, there is another window for that room that I could move the unit, but I would like to keep it open for aestethics (it is a front window) and to allow full ventilation of the room when the A/C would not be on (there is only one other easily operable window which is on the other side of that room.)