You can get UV-stable project boxes; see if you can find one in the size you need. You may also be able to route your coax up and underneath a roof tile and through the roof covering rather than drill a hole in a wall, and have it inside afterall.
Just about any weatherproof electrical box will work. You can find them at home depot or Lowes if you want to check sizes and see them in person. I have a metal 6inch square one on top of a tower in the sun and it has a cooling fan on the pi as it can get a bit warm on the CPU. So far my max is 61C this year but not bad considering it is running about 50% load almost 24/7. I just leave a hole on the bottom for wires and a little airflow and mount the pi a few inches up from the bottom and have not had any problems from water.