This model can be used to create fittings of any size between pipes of different diameters.

It is possible to parameterize the two ends of the fitting independently, as well as the central separator.

Below is the diagram of all possible parameterizations of the model.

And these are the controls that can be used to modify all the values.

Among the parameters, you can use the "preview" checkbox, which allows you to display the fitting cut in half, making it easier to properly assess all wall thicknesses.
It is essential to disable the preview before downloading the model for printing.

Print settings

To ensure that the printed fitting has sufficient mechanical strength, it is absolutely necessary to print it horizontally with some precautions: the aesthetic result may not be the best, but the part will not break. 

Of course, it is essential to use a water-resistant material, such as ASA or PETG.

I use these settings:

For convenience, I have included a print profile with the recommended settings that can be used as a template.
Simply download the model with your custom parameters and replace the one included in the template.