For sealing you can eather use a o-ring gasket D20x3mm or print the gasket with tpu or flex material. The gasket has to be placed on the bottle side inside the thread-hole

Supports are only needed for the outer ring, i used a supportblocker cylinder in the slicer program to avoid supports on the rest of the adapter.

I testet it on Sodastream EASY, TERRA, ART, POWER and JET which uses normally 1 Liter Sodastream PET-Bottles. 

For Sodastream DUO, please use my remix of this model:
“Sodastream bottle adapter V2 (DUO) for standard 0,5 liter pet-bottles”

Multifunctional because of the quick connect ring AND the thread connection for older Sodastream-Systems.

You can use a normal Coke / Fanta / … or other 0,5 liter bottles with this adapter.

I used as material for the adapter PETG, but also PLA is possible.

The quick connect is sealing on the inside, sandpaper the inner side of the adapter for better sealing surface quality if needed.