If you are interested in writing your own modules, this repo is a good starting point. You can copy it and start editing it to your liking.
You can use this module, or your copy of it, without submitting it to the index:
cfbs init
cfbs add github.com/cfengine/module-example
cfbs build
This also enables you to work with your own personal modules, they can just be in GitHub repos. (You don't need to ever submit them to the index).