Hi @Julien Recan, your cheapest and best option is to zip tie it. You can use 3 zip ties in a row to make it rotating. I am sure you will figure it out once you put the first zip tie by what I mean that you need 3 connected to each other.
A more fancy way of doing it which customized flags use are Rubber lined P clips. Offroading Flag poles usually need the 3/8" size. Here is one example of what I mean:
https://www.amazon.ae/HSEAMALL-Stainless-Steel-Rubber-Clamps/dp/B07RVJYTQY?th=1
In general, the fastest, efficient and economical way is to just zip tie it.
Hope this helps.