What an excellent landlord you are PW. 20 nest boxes, I kinda imagine you sneaking round while folk aren't looking and putting up a nest box on their patch!
I haven't as much spare wood as you (nor as much patience) so I admit to buying this robby box off ebay. I particularly liked the slate roof. Yesterday I put it up - on next doors fence too!
There's not a lot of vegetation around it - the honeysuckle hasn't yet grown, so don't know if robby will fancy it.
Edit - I did have to drill a couple of small holes in the bottom of the box in case it got water in, so it could drain.
OPEN FRONTED NEST BOX