I have been designing, building and placing nest boxes for American Kestrels for over 30 years, both in Wisconsin and in Colorado.
The nestbox dimenstions that most people use too small...in my opinion.
Therefore, I have a box design that works very well for the birds.
The dimesions are...
Side 7 x 11 and the lower part 9 x 11. This is the door placed to extract the young for banding and for cleaning the box
Bottom 11 x 9.5
The entracne hole is 3 inches and palced either 1 inch or 1.5 inches from the top fromtof the box.
The door is on the side and a nail is used to keep the door closed. Just drill a hole in the front so that a nail can be used to keep the door closed.