You could find an AC that fits between studs on 16" centers and cut a hole in an exterior wall.
A vented skylight is another measure you can take.
You might also try a duct booster fan, but the Stack Effect [hot air rises] is fighting you while trying to cool this area. This same effect will help some with the heating.
Heating & Cooling Degree Days - Free Worldwide Data Calculation
your Cooling Degree Days are about the same as mine, 1500. My area gets very humid in the summer which is an additional burden on the AC; I don't know about your humidity levels.