Adjusting the Offset

Open the blend file in Blender 4.2 or newer.  Adjust Value in the Shape Keys menu. Changing Value by 1 will offset each mouth by 1mm. Setting Value to 0 will basically reset the duct to its original shape.

Export your modified duct using File → Export → STL (.stl). Make sure Scale is set to 1000 and save the file somewhere you'll find it.