As I described in the one picture, I manually crop the wide image square (without cropping the one (shorter) dimension). It’s a time consuming process but can be made quicker by loading dozens of images into Photoshop at once, locking the crop aspect ratio to 1:1 and using keyboard shortcuts to save/close the images.
One of the developers got back to me elsewhere and said that manually cropping is a bad idea because that can’t be replicated at runtime. But what really can’t be replicated at run time is taking my widescreen camera image and squeezing it down to a 1:1 aspect ratio. 
(I.e. Training a model with a widescreen-source image using Stretch to: 640x640.)
FWIW, looking at these pictures again, those bounding boxes I used in the examples are terrible and way too big. I am pretty proud of my model though, it can be very accurate with only a couple hundred images per class. Not exactly “degraded performance” imo.