Here's an update: I bought a purple martin house pole, because apparently purple martins like to be really high, so it would allow me to get the box at least 12 feet off the ground. It came in 4 - 4' lengths, to make it easier to transport and work with, and had a sleeve to sink in the ground so I can remove the pole but leave the base in place if I want to. I sunk the 12" sleeve in concrete in the ground, which took about 20 minutes (including digging the hole) and was far easier than I expected. The hardest part was figuring out a way to attach a box to a round pole, but with some imagination and trips to the hardware store, I got it sorted. The pole is actually telescoping, so the piece above will slide into the piece below if I loosen the connectors (also handy).

The bat box is one I bought on line, and painted black so that it captures more warmth from the sun.