These are the two which I heard are perfect fit for the Ninja 650 Stem hole. Haven't tried them myself due to the Illogical pricing.

https://www.amazon.com/Mounts-Lab-Motorcycle-Compatible-ML-Ninja01/dp/B09VGQPGFG

https://www.amazon.de/-/en/gp/product/B00M0T1WTM/ref=ox_sc_act_title_2?smid=A3PT4HPCTKFK3C&psc=1