Vagrant providers each require a custom provider-specific box format.
This folder shows the example contents of a box for the google provider.
To turn this into a box:
$ tar cvzf ../../google.box ./metadata.json ./Vagrantfile
This box works by using Vagrant's built-in Vagrantfile merging to setup defaults for Google. These defaults can easily be overwritten by higher-level Vagrantfiles (such as project root Vagrantfiles).