For mounting ducts to flat surfaces. Suitable for ventilation ducts, cable management ducts, etc. With optional holes.

Open the .scad file in OpenSCAD and modify the variables in the beginning of the file according to your needs. Everything is in millimeters, except for ‘makeHoles’, which is “false” if you don't want holes, or “true” if you want them. 

Press F5 in OpenSCAD to see a preview of your modifications.

Once you're done, press F6 to make OpenSCAD compute the geometry. Wait until it's done, and then press F7 to export STL for your slicer of choice (PrusaSlicer, Cura, Simplify3D,...). 

Print with the base facing down, no supports, add a brim or raft if you're worried about warping.

There are also finished STLs for 100mm and 125mm ducts available in the Files section if you don't fancy tinkering with the variables.