Couple different options on Etsy as well, they’re a popular product I guess!
https://www.etsy.com/listing/1177629606/tentclips-grow-tent-fan-mount-works-with?gpla=1&gao=1&
https://www.etsy.com/listing/1182414171/grow-tent-fan-mount-6-8-inch-tent-poles
https://www.etsy.com/listing/1055075248/grow-tent-fan-mount-12in-16in-hurricane