Oops, my bad. I totally misremembered the create map set & download wasn't through OSM, but it a tool that used OSM as the source.

I have since retraced my steps from my original memory of DC Rainmaker. And see that following DC's link listed in "Part One: Downloading the Maps" to the OSM BBB map creation tool yields a warning and an automatic redirect to here: https://garmin.bbbike.org/

Sorry about my faulty memory. My Edge 520 has not needed a manual map update in many years. Connect does the job annoyingly well.

I too am having trouble creating a bounding box on my mobile device. But I expect a regular computer with mouse would have better luck. I'm actually a little interested because until it's a thing I always took for granted as a thing that would continue to be straight forward & simple to do.