At least change the direction of the entrance hole so the hawk can't see the entrance from his likely perch. Thank goodness this haven't been a widespread problem. Huber Venice Fl. Sure, lenghtening a roof here and there may make it more difficult for raptors to capture something from within a box. But, ultimately, their patient persistence will win out and sooner or later they will capture their prey as it moves to and from the box.
Raptors are patient birds with regular habits and routes of predation.
In my opinion, moving a box which has become a target for raptor predation is the only sensible solution to this age old problem. You do not have to limit yourself to ground-mounted nestboxes. Hanging boxes can be moved anytime regardless of frozen ground. They are beautiful birds but all the regular yardies leave rather quickly upon arrival of the Kestrel. I think I would die if I had to watch them swoop down and pick off one of my blues.