We have recently made the Endless OS Image Builder publicly available. This is the tool that we use to create installable disk images. You can get it at https://github.com/endlessm/eos-image-builder. This is aimed at technical users or organizations that have a need to build their own custom images.
We’ve tried to streamline the tool so it works out of the box, but feedback is welcome!
Instructions for using it can be found on the support site.