Simple ring tensioner, inspired by https://loopalien.com/
It's usable in many different ways, please don't use it for climbing or other “vital” tasks.
The first 3 pictures shows the 15mm version with paracord 550 (7 strings, about 3,5mm diameter).
Paracord on amazon.de (affliate link)
5 sizes:
for use, watch these videos: