Do you have trouble with your boxer in the heat? I was advised on the boxer board to not even think about getting one, because they have heat exhaustion starting in the 80 degree temp range> because of their short snouts. We live in Texas. Usually not the high humidity, although the weather has been changing over the last several years to more humidity during the summer, usually its a dry heat. We do get hot. By August it is 100 deg. every day.
I've only lived in Maryland and Germany with Reggie, and in Germany it does get into triple digits during the summer. I have packed with him in the higher heats and as long as I kept him hydrated and cooled off with water he did great. The ruff wear packs also have a model that has built in water bladders that help in keeping pets cooler. They are the Palisades Pack II link In Maryland the heat doesn't get up to triple digits but it gets into the 90's with VERY high humidity.
Reggie never had a problem. I have always hiked/packed/trail walked in the woods though so it was cooler, and with the shade the ground was always cooler for rests.
I lived in San Antonio for about 18 months (8 yrs ago) and I had a pit mix that had no problems when we went out for walks and trailing and he was all black. I just made sure I kept enough water for both of us.
They also do make wonderful cooling items now too, they have neck ones and belly wraps.
I'm gonna PM you about boxers