My old design worked quite well but was a little bulky. As I just discovered the sculpt features in Fusion 360, this one was a lot easier to design and looks much more light weight.
Work in progress as I think I will have to tweak it a little bit to be able to print it without internal support.
Update: Added a new version as the first one ("V2") didn't have a hole in the mouth piece anymore after printing due to tolerances. This one works great.
If you don't have a fitting O-ring, you can just wrap some dental floss into the groove to create a sealing. Works great for me.