I made a fan mod for my Raise3D N2 Plus Dual (this should fit all N series I think), I wanted to add a 40 mm fan to be able to control the airflow on the printed model using gcode.
In this setup the stock 35 mm fans only cool the cold end and an extra 40 mm fan on the front cools the printed material under both nozzles. You can connect the 40 mm 12V fan on the extruder board next to the two ''always on'' fans for the cold end.
Make sure the heating elements don't touch the duct - I had to move mine a bit to the back in the hotend but it fits well.
I added the STL files and an Ideamaker file for the 40 mm duct with added support. The files with ''V2'' are updates with smaller holes for attaching the front duct to the two outer parts, in stead of 4 mm the holes are 2.5 mm in diameter so they should fit an M3 bolt tightly.