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I use this one:
https://www.amazon.de/LICHIFIT-Sattelst%C3%BCtze-R%C3%BCcklicht-Halterung-R%C3%BCckfahrradar/dp/B0832LRC69/ref=sr\_1\_1\_sspa?\_\_mk\_de\_DE=%C3%85M%C3%85%C5%BD%C3%95%C3%91&crid=2O5Q1BG4IAT44&keywords=varia+radar+mount&qid=1654423143&a...
It's a mashup of GoPro mounting arms and this sort of thing: LICHIFIT Bicycle Saddle Support... https://www.amazon.ca/dp/B0832LRC69?ref=ppx_pop_mob_ap_share