Keeping Squirrels out of Owl Nest Boxes

I've been lucky this year and had no problems with squirrels in my screech owl nest box. Significantly, I think I know why.

Upper-left: Screech owl nest box in gray.Middle: Fox squirrel nest box in green.

Though I've seen fox squirrels taking an interest in my squirrel nest box at various times, for lack of built-in video cameras or other instrumentation, I remained unclear as to what, if