While in the process of installing a CO2 air filter on my fish tank I realized I lacked a tube adapter to join two tubes of different diameters. Less than one hour later the problem was solved :)

You should be able to customize this to choose the length and diameters of the small and big ends, the number of steps and the increase between the bottom and top of a step.

You can do that with Thingiverse's customizer: https://www.thingiverse.com/thing:3400024

Beware, I have designed and used this for tubes that only carry air under normal atmospheric pressure.

You will need to adjust diameters depending on your needs, and it might not be a good idea to use it for critical devices.

DISCLAIMER :

I can't be hold responsible if it fails to carry whatever you might use it for (water, booze, whatever), and the damage that could result of a leak or whatever dumb situation your poor judgement might put you in :)