Well we think it's pine marten proof. To get into the nest the pine marten would have to walk upside down below the ridge then drop on to the owl's landing pad so we think any owls will be safe.
First find a 45 gallon ( 100 litre ) blue plastic drum. cut out an entrance approximately 6" x 6" (150 mm x 150 mm). Make a landing pad about 6" square then attache to the front of the nest with two roofing bolts. Finally drill a series of drainage holes along and through the lowest point at the bottom.
Attache the nest below the ridge with loading straps strained tightly the nest box must be rock steady.
You may now have to wait several months or years until an owl takes up residence but just to see if last night's owl is interested I have set up the trail cam.
More tomorrow perhaps.
If you do get a breeding pair in this box you can use the white plastic plugs in the end to insert a small nest cam.