Hi, I have successfully applied a sandbox to Steam thanks to the sandbox utility that Portage uses to safely emerge new packages. This tool allows to easily set an environment limiting the access to the filesystem on a selective way. Combining the sandbox with the $HOME and $XDG_DATA_HOME environment variables, Steam can be fully contained in a single folder that holds the steam client, game files and also the full mess of saves and config files. This setup does not put any more order on the constellation of files created by steam and the games, but at least it places them in a single folder away of your home. A short guide with instructions is available here:
http://nthloop.blogspot.be/2015/07/simp ... entoo.html
I have tested it for a few months and it works well. Steam and the games play and update as usual. The only issue I've found are a few games that search the home folder based on the user's name, and do not follow any env variables. These games will fail to start. However, it is easy to spot what files/folders they are trying to create outside of the sandbox and if necessary, it is also possible to create symlinks in place of those files/folders to somewhere inside the sandbox.
If you try it out and find any errors on the guide or problems with the setup, please let me know.
