After researching owl boxes,
we decided to go with a box with the following features:
- an internal wall for protection from the opening
- a lid that opens
- a side flap for easy clean-out. 

Demensions of our boxes:
- 5 inch entry circle
-3/4 inch plywood used
-17 inches tall
-23 inches wide
-11 inches deep
- mounted at least 8 feet above the ground