Summary

Easy to use.
The slotted beam pushes half of the chain yarns down, so the spool can slip through these yarns. Then the slotted beam is rotated 180 degrees and the other half of the yarns is pushed down.
After each cycle, the beam is pulled to the fabric to properly align the fill yarns.

https://youtu.be/9WS4un1w-XY

The video clearly shows that I am not an experienced weaver...
Thats why the next version is easier to handle:
https://www.thingiverse.com/thing:3177236