I can't find anywhere that tells me how to do this
I have several medium sized trees in a woody area which I think is the best site - and I have 2 boxes ( not sure what they are but they don't have round holes so I suppose they're bat boxes) & I can't see how they're fixed

Is it Ok to nail/screw - I'd prefer not to - & if not then how?

I have a shed with eaves which is quite high off the ground - but is it an issue under the eaves where things can reach down from the roof? ( they could get access via next door's oak tree)