There's also Firejail http://sourceforge.net/projects/firejail/
and subuser http://subuser.org/ that are worth a look.
I currently use sskype for sandboxing Skype, as it is the only non-free app on my system I do not trust. I wanted to transition to sandfox.
https://wiki.archlinux.org/index.php/Skype#Securing_Skype
I will definitely check out subuser stat. The idea of coding an alternative using LXC or systemd-nspawn and Xpra seems like a super appealing first project I could really get lost in when things slow down for me!