This adapter converts 2x120mm square fans to a 4 inch female air duct hose. It is meant for feeding air from 2 side by side 120mm computer fans to an exhaust air duct for from a 3d printing cabinet. The 2 120mm fans should be mounted to a MDF board with 2 circle holes cut out for fan air flow. This adapter should be mounted on top of the 2 120mm fans using the four mounting screw holes on the sides that will pass through the fans' mounting holes. You may need to buy extra long screws for mounting. I am using an IKEA Sektion cabinet with MDF board cut as "shelves" dividers.
I use this with https://thangs.com/mythangs/file/195586 to exhaust air from my printer cabinet.
The full model is dual120mmto4inch.stl. It has been cut into 3 smaller pieces for printing on an Ultimaker size printer. These 3 smaller pieces can be oriented to print with no support needed.