Pre-compiled

gmaptool.eu provide pre-compiled OpenTopo maps for large areas, including the US (which seems to be a rarity).

Bring/build your own

After moving to the US I found it hard to find a place that provided map tiles that are supported in BaseCamp.

Ended up finding the dev process for freizeitkarte in their Github Repo and folling it. Some more info on the process at https://www.freizeitkarte-osm.de/garmin/en/development.html.

To build a .gmap, an expected directory structure, for BaseCamp, do the following, then move the .gmap into the BaseCamp Maps directory.

Supported out-the-box regions for their tooling can be found here.

It may well be possible to create a custom shape and feed it into their system but I did not end up trying this.

On Linux: Each step can take a while, overall process took approx 90 minutes for the US WEST region.

region=Freizeitkarte_US_WEST
ram="--ram=16384"
cores="--cores=max"
perl mt.pl create $ram $cores $region
perl mt.pl fetch_osm $ram $cores $region
perl mt.pl fetch_ele $ram $cores $region
perl mt.pl join $ram $cores $region
perl mt.pl split $ram $cores $region
perl mt.pl build $ram $cores $region
perl mt.pl gmap $ram $cores $region

### Optional: for windows installer, not needed but I did try it.
# nsis only supports up to 2GB
perl mt.pl nsis $ram $cores $region