After many tries, I found that solution (who works !!! :o))) ) in which the bottle neck himself ensures the airtightness.
Designed to fit on german (and others ?) refundable PET bottles.
This is my second version of the Sodastream PET bottle adaptor. A friend of mine has been in trouble as her adaptator stayed stuck in her SodaStream machine. So, I drawed a new one with an extraction tool.