Every time I build a new AR, I typically paint the receiver set. I have always just used blue painter's tape. But I decided to make it a bit easier. I made some 3D printed masking inserts. I even added holes on the ends to hang them from a wire to so I don't spray my fingers and to hang them in the shop to dry/cure. I typically use either Brownells Aluma-Hyde or Rapco rattle cans.  

There are a few screws used to hold the assemblies together.  I made the holes to accommodate 3mm screws, because most people with printers have a plethora of extra 3mm screws in a parts box.  The forward assist and trigger guard pieces are held in by just a small length of 1.75mm filament.  adjust as you see fit.  I included STEP files to make it easier for anyone to use/mod them.  I also made a second version of the lower main mask.  It is for if you strip down a lower that is already built and don't want to bother with tapping out the pin to remove the bolt release ping pong paddle.  I think that's it.  Enjoy!  Prosper!