To make the tinkering easy, I've made this script.

What it does:

1. Try to find the game folder
2. Create the eac.so symlink
3. Edit the ini file
4. Try to launch the EAC Runtime installer on Steam.

if you have the Fall Guys game installed inside the default Steam Library location, just run the following command:

curl -s -L https://gist.github.com/rokam/3fb8b768606eef1ddd696cd0bc02e6df/raw/6a0f3a502b2722c87e92ea9cffb8a00144f8a76a/fix_fallguys.sh | bash

If you have it in a custom location, replace the /path/to/your/SteamLibrary/steamapps/ with your steamapps folder containing the game files:

curl -s -L https://gist.github.com/rokam/3fb8b768606eef1ddd696cd0bc02e6df/raw/6a0f3a502b2722c87e92ea9cffb8a00144f8a76a/fix_fallguys.sh | bash /dev/stdin /path/to/your/SteamLibrary/steamapps/

Known issue:

If your Proton is installed in a different Steam library location then EAC Runtime, Proton won't be able to find the EAC Runtime. This seems like a bug in Proton.