This is one of Scott's photos (used with permission) showing the very slanted and linoleum covered roof of his "SQR" box. The pair of adult Screech Owls sitting in the box nested at this site this season.
Scott has had great success with his SQR design, keeping the squirrels out of the boxes with the linoleum covered sides and roof along with the correct placement of the box to keep the squirrels from jumping onto the front. Scott has directions for placement and maintenance of the boxes on his Etsy store along with tales of his success at the 9 sites he has erected boxes on his blog Owl Oasis Trail.
Here's another of his great photos showing a pair of young owls peering out at the world.
I've had my share of squirrel problems over the years and currently use baffles either on the tree or pole the box is mounted on with varied success and additional cost. I plan to try a pair of Scott's boxes, one with the slotted opening and one with the 3" round opening in the woods behind the house and am looking forward to reporting the results! Great idea Scott!