An opening needs to be made in the cap of the PET bottle to accept the old spray can nozzle. For my paint can I found that an opening slightly larger that 3/8" was required.
I tried a few different types of adhesives here and found that the best results were with Sugru
. I was able to mold the sugru putty around the opening and join the old spray mechanism with the PET cap. Allow to cure overnight.
After your assembly has dried and cured completely hook it up to a bike pump and test to ensure you have a hermetic seal, if you have any leaks (you'll know) you need to address this with more glue or to re-glue the parts that aren't sealed. This is critical!