We can use gmapping model to generate the map file: **.pgm and **.amcl, the latter is just a refer to the **.pgm map file.
Here I introduce how to use the image editor "" to modify the **.pgm file to meet our requirment.
We just used gimp. It's just a bitmap, so we drew some extra black on there with the paintbrush. You may think that gimp is too big size.↓
Here is some substitutions of the big-sized pgm file editor gimp:
Krita
It's in universe ubuntu repos and is lighter than gimp. I recommend.
Inkscape
Inkscape which seems to support .pgm extension (disclaimer: I haven't tested it). Install it with
sudo apt install inkscape
Netpbm
This page reports that netpbm can be a cli solution for *.pgm extensions too. Try it. This is the homepage of Netpbm.