I looked into the type of thread that was used for these types of tubes. Everything pointed to a S15x2.5 specification; which is a type of metric buttress thread profile. Seems to fit everything with the same tolerancing that store bought adapters do! Enjoy!
There is a blank version and three other versions featuring letters C, G and S for caulk, glue and silicone.
** 1/1/2025: Added versions with a center plug ** (See Drawings, (2) - versions)
The author marked this model as their own original creation.