Here is a simple cap to keep the oring in place and dirt largely out of your regulators.
as with other threaded items sometimes a little scaling is needed to get the threads to fit easily.

I have added a second design to these with the hole for the string to the side so as to allow the cap to be better sealed to dust ect.

I printed mine at 2% larger and flipped them on the x axis 180
there is a slight taper between the nut and the body to aid in printing them.