How to install surface mount box under eave with aluminum covering
He has an old work box. You have exterior grade; you can put that wherever you want. If you're worried, use dope on the knock-out plugs and the fitting.
Drill a hole in the eve right up against a joist (1&quot; hole saw, or however big it needs to be to actually fit the fitting - it's always bigger than you think). Put the wire through the hole and into the rear KO, then angle your screws to grab the joist. Or add some framing to catch a screw from the other eyelet. I wouldn't caulk anything.
In the future, always plan the end point before you begin.