As recommended by the Xerces society, I drilled holes of several diameters, as different species tend to be fussy concerning the size hole they choose for depositing their eggs. Each species seems to prefer different materials for sealing up the nests once eggs are deposited, such as wax, mud, or plant material.
That is born out by the bees that found a home in my hive! Yes, I have bees!
Here is a pic of my do-it-yourself hive:
A close-up of some of the holes:
In the upper left, you see one hole filled with a brownish material. You also see that in the hole in the extreme lower right. At first, this substance was green, appearing to be chewed-up plant material, but over time, it turned brown. Underneath that hole, the 2nd, 3rd, 4th, and 5th are plugged with a waxy polymer sort of substance. The 3 holes starting from the upper right are plugged with mud. If you look carefully, you'll see the 3rd hole has a tiny opening. Bees have natured their and burrowed out.
When I first placed my hive up, it was shaded under a bush. No bees came. After giving it a couple of weeks, I moved the hive to my porch. There it got a couple of hours of sun, face on. Within just a few days, bees showed-up.
I know you can buy these things pre-made. But all I've seen only have holes of one size, so you will be only attracting bees which prefer that size. You'll be helping bees, which have been under a lot of stress lately. If you have children, have young friends, or are a teacher, this is a great way to encourage and interest in science and entomology.
Few things bring one closer to the Goddess and God, than the study of insects.