Go to https://geo.javawa.nl/srtm/index.php, select the region you are using and download each file listed on the right (use the lowest resolution available)
Extract and move the contents of all height maps to the DEM directory
Edit mkgmap.args and replace map_with_dem_files with DEM
Draw polygon around desired area. Select KML in the lower left corner and click the copy to clipboard icon
Paste the KML data into a new file (it is not actually in KML format). Edit the ploygon file to list coordinates 1 per line, start each line with 2 spaces, replace comma (,) with 2 spaces