For CGA 320 (aka CGA320, CGA-320).

The dust cap is threaded so it doesn't fall off easily.  It has a finger-friendly hex-like shape at the back for easy turning.

Some might prefer a remix that doesn't have threads and instead relies on layer lines to stay on but can be pulled off all at once.  Not sure how easy it'd be to tune that to fit well enough, but the .f3d file is included in case anyone wants to remix.

I think PETG is a bit better for this than PLA, if only for the better impact resistance if somehow the cap gets bumped into something.  But either should work fine.

For slicing, Arachne works great for this (PrusaSlicer 2.5 or Cura 5).  Pre-Arachne might not work so well since it's thread on the inside of a thin wall.  Yay for Arachne!

The 103 percent in xy is to loosen up the threads a bit.  At 100 percent, it's a challenge to screw it on more than a few threads.  At 102 it'll go on there, but way tighter than necessary.  At 103 percent it's just loose enough to turn quickly.  At 104 percent it's a bit wobbly but still works fine.  The STL has the 103 percent in xy baked in so hopefully it'll just work for most.

0.2 layer height works to print the threads - probably 0.3 height would struggle.