With this design you can produce a sleeve to connect hoses or pipes with different diameters.

The design is parametric and thus fully customizable to fit your needs. To adjust the dimensions, open the FCSTD file with FreeCAD, then the 'Dimensions' spreadsheet in the tree view. You can have both the design and the parameters side by side by clicking on the 'Windows' menu then 'Tile'. The parameters names are self explanatory. If you don't need flanges inside the sleeve, set the Flange Thickness value to 0.001 instead of 0 as it can prevent FreeCAD to make the finishing Chamfer and Fillet. Once you are satisfied with your settings, export the resulting sleeve in STL. When some parameters combinations are impossible, FreeCAD will either ignore your choice or just crash.

I provide an STL file as example.

This is an open-source design. FreeCAD file is provided.
Feel free to modify this thing at your convenience.

Good design is hard work, even small donations are welcome.