If you need a different height for the ducts, because you have a different length toolhead installed, or you just want to play around with the geometry of the outlets you can import the f3z CAD file to Fusion360 and by just editing one sketch achieve your goal.
Select File > Open... select the duct you wish to edit and upload it to Fusion360. This will take a little of time. Then Open the Project File.
To adjust the height of the Outlet change the highlighted 3 mm dimension. 3 mm is the value to target the nozzle height of the Mini StealthBurner. So if your nozzle is for example 2.5 mm lower you'll have to add this to the 3 mm: 3 mm+2.5 mm=5.5 mm is the new Dimension you want to enter here. Afterwards click finish Sketch and wait for Fusion to do all the calculations.
Generally the Yellow Warnings can just be ignored. These often come from some edges on chamfers/fillets getting combined and not needing a chamfer anymore.
If you get a red Error you will have to check in the timeline what operation is causing the Error. These usually also come from chamfers or fillets, these can most of the time be fixed by deselecting the edges in question.
Now Export the parts that you need as STL's.
Important For the ducts export the components that contain the support structures as one file as otherwise the supports will be missing after the export.