The Stihl Filling System is great. Screw it to the petrol can and fill your chainsaw without spilling a drop. But the system will only fit to Stihl cans with quite expensive fuel.
I found another fuel and first used a funnel to fill it into the stihl can.

Now I made the adapter, which works fine.

Please note that fuel can be dangerous - especially fuel gases leaking from the can!

UPDATE I added a version2 with more "grip" - and - as I tried printing TPU - I added a gasket as well. It´s much easier to get it tight with the seal. You can proof it by gently pressing the can: You will hear the air passing off, if it's not tight enough. This is importend: When it´s not airtight, the system will not work well.

I made mine from PLA and I am not sure how long it will last.
So another warning: Use it as careful as I do. Don't leave it on for a longer time and check it's tightness and stability everytime you use it.

OLD VERSION There is a gasket in the filling system that works, but non on the side of the adapter to the can. Nevertheless mine was airtight. I just hat to screw it very tight to the red can. You can proof it by gently pressing the can: You will hear the air passing off, if it's not tight enough.

There is a version with modified diameter for an oil cannister as well:
https://www.thingiverse.com/thing:3411577/