Remake of krazydarcy's adapter (https://www.thingiverse.com/thing:1840408). I printed his first and took measurements on it to match the threads, but tried to match the dimensions of the gas mask to make the adapter smaller.
V2_5 is tested and fits quite well!
The threads are large enough that 0.3mm layer height might work. Untested.
The body is a revolution about the origin, the threads were added by sweeping a rounded triangle up a helix. Thread pitch appears to be 4mm on both ends.