Where can I buy the C-clamp (mounted from above, or L-profile clamp according to the video) as an accessory?
https://media.ergotron.com/reserved/resources/choose-attachment-lx-de-orig.pdf
I was able to carry out a search and unfortunately could not find any mounting kits for the desired C-clamp for the American market. The manufacturer may be able to send such a kit directly, in which case I would recommend contacting the manufacturer directly.