These are the modelines I used for 640x240:
xrandr --newmode "640x240" 13.220 640 672 736 832 240 243 246 265 -HSync -VSync
xrandr --addmode DVI-0 "640x240"
xrandr --output DVI-0 --mode "640x240" --scale 1x2
You can modify the 15khz-x11.sh script to change them to those. Just remember to keep a backup of the old one in case it doesn't work for you.