Oh, Sam, don't do it, if they are going to be on an outside wall. First, I once opened a nestbox to check on the babies, and a baby flew right out past me. It was in my birdroom, but still took me over an hour to find it. If it had been outside, I probably would have lost it. You think you can catch a little one, but they are surprisingly quick, and since so many of them don't have much color at first, they are hard to see.
Second, if you have critters around your place (everyone has critters of some type), a nestbox on the outside would be easy pickings for a racoon, opposum, or snake. I had a snake get into my shed where I used to keep the birds, it crawled into a cage, ate 3 juvie Stars, then I caught it because it was too fat to crawl back thru the bars (it had curled up in the middle of a plastic plant for a nap).
If you have a secure entry vestibule into your aviary, then you could maybe mount a couple of nestboxes on the outside of the aviary but within the vestibule. Otherwise, I would try to hang the nestboxes inside the aviary but not so high, so I could reach them to check on them.
