This interlocks with the existing slots and clip. It attaches to the base of the soda stream. Start by tilting it back, then slot it onto the rim at the back of the soda stream base, and then tilting it forward into place. It slips on without resistance, if youre forcing it, youre doing it wrong. 

Print it upside down. (It is already in the correct orientation to print in the stl file). It needs a small area of support for the arch way. 

I have included a prametric fusion 360 file if you need to adjust the height. 

Please let me know if ofther heights are wanted. I'm happy to upload more height options.