Completed this on July 11, but forgot to post pictures. Thank you to everyone for your input!
Original wiring
With the Sonoff wired in between.
Installed and working, before being hidden in the can.
The pictures don't show it, but I replaced this version of the Sonoff with ZBMINIL2 so there is no issue with this acting as a repeater and killing other devices when the main switch to the lights is off.
