I thought I would need to make an eccentric pipe reducer recently for a new cooker hood ductwork. OpenSCAD's cylinder command doesn't allow the axis to be skewed so I didn't model it. Fortunately we were able to fit the ducting without it.

This model allows the user to specify two pipe inner diameters and wall thicknesses. The upper pipe can be offset by a percentage of the radius (0% is concentric, 100% is fully eccentric) and the centre can be rotated if required (though both axes run vertically).

Finally, the gap between the two pipes can be specified and whether the inner wall or outer wall should take precedence for modelling. With 100% offset and a narrower upper wall, the pipe will run through if the inner wall takes precedence. There would be a step in the outer wall. If the outer wall takes precedence, the opposite would be true. (Its easier to see than describe - have a play!).

Edit 01Feb22: Thingiverse haven't fixed customizer. I recommend that you install OpenScad if you want to customize.

Edit 25Jan23: The new Thingiverse staff have fixed customizer. Yaay! Well done team I still recommend using OpenScad, but you can use TV's customizer too.