I got one built by a good friend of mine after hearing an owl hoot in Fall a few years ago. We downloaded the plans from the web and used cedar to build the box. He mounted it ~20ft up on a PVC pipe and tethered the pole between a pine and a mulberry. Except that the owl never showed up-actually I haven't heard it again. But in the meantime, gray squirrels enlarged the hole and moved in. They have been living there ever since. I figured it is giving shelter to some creature especially in brutal winters such as this one and let it be. Squirrels are quite smart and will take advantage of any structures given half a chance. Mounting your box away from trees and securing it to a wooden pole covered by a smooth PVC pipe/sleeve might do the trick.

Yes, given by the size of the entrance cavity, the cavity was probably originally made by a Northern Flicker. BN

I don't know how deep the nest actually is because I can't get up into that tree. From what I've been able to see, however, it appears that they are really exposed to the weather more than I ever expected them to like for nesting. There was once a very large limb that broke from this tree and eventually it developed into this large cavity. You can still see some of the limb at the top of the nest site. That area of the tree has really deteriorated over the last few years and I worry that it will soon be too open for the owls to continue to nest there. The tree is slowly dying and is losing limbs every year. Many sections of the tree have holes in it made by Pileated Woodpeckers. Squirrels and birds fly into one hole and out another hole in another section of the tree. Definitely hollow spaces in there. I've been contemplating making a nest box and mounting up that tree in that nest area for next year. I'm just not sure what to do. I've watched them nest there every year since 2003 and I would truly miss them. I want to offer a nest box but at the same time I don't want to spook them away from the site they have used for so long. Here is an image from 2003 when the young were leaving the nest and the tree is full of foliage.

Three years ago I built a screech owl nest box and put it in a maple about 15 feet above the ground. After a starling nest and two years of squirrels, a family of red morph screech owls have moved in. The tree is at the edge of a wooded area facing south toward a 20 acre area of open organic farm, some wooded area and open field. The fledglings can often be seen peaking out of the hole. I live in eastern Long Island NY.