Convert OpenStreetMap / OpenMTBMap for Mac

Download a map.osm data file using the openstreetmap export tab - it only seems to permit small areas.

This can be quickly converted into an img file by
java -jar mkgmap.jar map.osm

But for Roadtrip file you need to some more steps, but still using mkgmap.

java -Xmx512m -jar mkgmap.jar --tdbfile --gmapsupp map.osm

--tdbfile = requests a tdb file (a Garmin format for summary information) as part of the output
--gmapsupp = requests an img file (Garmin format to copy as gmapsubb.img to Garmin when connected via USB)

This should result in 2 img files and 1 tdb file.

These need to be fed into gmapibuilder. It is enough to use the GUI to find the tdb file and the rest is automagically filled in. Pressing "Convert" results in a gmapsubb (presumably just a renamed version of 63240000.img and an 'OSM map.gmapi' file.

OpenMTBmap promises some nice maps, but the instructions for converting to Mac format cannot be directly followed. Instead...