We live in the middle of a town. The middle mind, not the edge. SWMBO feeds the birds (excessively in my opinion), we have the odd grey squirrel and a few big trees nearby including a big London Plane in our garden that I want to die but has a TPO on it so I cannot help it along. SWMBO then saw a feature on owl boxes in some magazine or other and looked at their website and found the plans. Now it is easy enough to build, but the chance of an owl noticing it, let alone nesting in it given our location and that of the tree is pretty much zero at best. But for a quiet life I agreed to make the box anyway....
https://www.barnowltrust.org.uk/barn-owl-nestbox/owl-boxes-for-trees/
So my question for you more knowledgeable types is, where does one get treated plywood from?! Goggle doesn't seem to know and SWMBO vetoed the idea of one made of OSB, one wrapped in plastic or one with an offcut of EPDM roof (no imagination some people!). I reckon standard WBP ply would be fine but any thoughts?