https://github.com/Annex-Engineering/ANNEX-Engineering-License-Agreement
Please follow the license agreement above.
Mostly putting it here for my reference/people to see. Used my spare parts so it's not easy to build your own. You probably need to mod this to use different bearing/roller. Does use standard bmg gear.
Overall cost is around $20 from aliexpress.
When assembling, move the stepper up and down so that there is a tiny bit of slack (as little as possible). When fully pressed, mine started binding. Not sure if it's gear quality or not. Also apply lots of lube. Put a wash between stepper and mount on one side to fine tune as needed.
The author remixed this model.