I have received several inquiries about making homes for Mason Bees. Here are some useful references that may give you ideas. They are expensive to buy and really not that hard to make , just make sure you use the type that can be cleaned out at the end of the season, removing the cocoons cleaning out the parasites and storing them for the winter. Do not use holes drilled in a wood block unless the holes are large enough to have a tube fit inside of them, so that the complete tube is removable. The tube diameter that is best is 5/16” (8 mm) , but a variety of sizes close to this wont hurt.
Next winter I will harvest more of the Phragmites tubes so that I will have extras to sell. Then you just need the equivalent of a birdhouse without the front on it.