Proton-5.21-GE-1 GloriousEggroll

mkdir $HOME/src

cd $HOME/src

git clone git@github.com:z0z0z/mf-install.git

WINEPREFIX="$HOME/.steam/steam/steamapps/compatdata/983970/pfx" PROTON="$HOME/.steam/root/compatibilitytools.d/Proton-5.21-GE-1/" ./mf-install.sh -proton

This could possibly work in Native Proton 5.13, but since I was changing/tweaking prefix files I figured it might be safer to go with another version of proton.