David - Thanks for the information on the use of creosote and the link to the site.
We always use plywood (the type sold for external use) for our wooden Barn Owl nestboxes. For outdoor boxes we use 12mm thick ply and so far we have found this to be fit-for-purpose.
Most of these boxes are 750mm x 450mm x 450mm and follow the design contained in ‘The Barn Owl’ by Bunn, Warburton & Wilson. However we attach a baffle inside the box (opposite the entrance) to prevent the ingress of driving wind/rain into the nesting chamber.