No "how to" as such, but you might get some hints from our LXC host appliance. Community member John has been working hard (with some assistance from Anton) on updating our LXC host appliance. His fork of the code (which will be merged back into our repo once he's finished) is on GitHub. The script that downloads the existing tar.gz templates and prepares them for usage is this one.

THe idea of our LXC host appliance is that it is optimised for TurnKey LXC containers but should support any/all LXC guests. If you're keen then it would be awesome if you wanted to help testing. The release of the v14.0 LXC appliance is the last piece of the v14.0 puzzle!