The other day we went to visit some friends. My friend was in the garage taking apart some Ikea bookshelves they were giving to us.
We went into the house for about an hour to have lunch and talk. When we came back out to the garage, I watched a mason bee fly around a bit, then zero in on the pile of shelf pieces and disappear. I picked up the shelf where the bee had disappeared, and discovered she had moved in to a hole where a metal dowel normally goes to provide strength.
I knocked the shelf on the ground a couple times, and a bunch of pollen came out. It was clear she was planning to stay, so I waited until she came out and flew away, then put the shelf in the car.
I imagine there are cheaper mason bee condos, but she was more than ready to take up residence in an Ikea house!