I've made this a long time ago from some leftovers of clay.
For the decoration I used a leaf with fat veins.
After these blocks of clay were fired, they layed in our shed for quite some time before they got used at last :-)
The solitary bees use all kinds of material to fill the holes.
Broken dolomite from our driveway, white loam from our wall in the shed and all sorts of wax.