This is a 40mm fan to 5015 fan adapter. Please be awere not each fan has the same mounting holes. These are the fans i am using (including a drawing with sizes):
https://www.amazon.de/gp/product/B01JJ0QD6Q/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
Category: 3D Printer AccessoriesThe author marked this model as their own original creation. Imported from Thingiverse.