After printing the awesome Japanese Water Fountain / Deer scarer at https://www.thingiverse.com/thing:2875176 and getting some parts I found out that the tube I got was fine for the pump I attached it to, and the pump connected just find to the fountain base, but the other side of the tube needed to be way thicker.
The outer diameter of my tube was a measly 6mm where it should have been 11-ish for a proper seal. I didn't feel like glue/silicon sealant was going to fly here, so I designed this converter.
It inserts into the base where normally the 11mm outer diameter tube goes and adds a nipple to which my tube connects.
The inner diameter of the tube I have is about 4mm, which fits just nicely on the printed part.
Fusion 360 file included if you want to customize it.
20%, doesn't really matter
I used a 4mm brim to print this, because otherwise the bottom has nearly nothing to stick to the bed.
With the brim it printed just fine the first try.
Sanding and glueing
After printing I sanded the printed part a bit and also the fountain base hole.
Apply some glue and it should fit snugly, but be sure to carefully test-fit it and optionally sand it some more ;)
Design was relatively simple, just sketched a few circles with the requested diameters, extruded them upon eachother and added a few chamfers and fillets.
The part that the tube shoves over I added a revolved triangle which I patterned a few times, so it looks a bit like what you would expect from a tube post :)