The rings holds two 20x3 cm magnets that can be inserted through a slot.
A compatible magnet can be found here: https://www.amazon.ca/TRYMAG-Neodymium-Magnets-Whitboard-Scientific/dp/B09XV87SV6.
Epoxy may be used to close the slot up permanently but is not required.
Supports for printing are not required.
The author remixed this model.
Optimized ergonomics and added space for two stronger magnets.