I know traxxas had a ball diff years ago, and if you tighten it to tight it broke or didnt work somehow. I never bought one, and until you explained, I never knew they slipped. can't say if that was hearsay or not.
They also have lockers on ebay, aluminum and steel. You pull the diff, pull the 2...
On the scrap piece of plastic, try different fine sand papers and wet sanding. That creates a flat look, but if not done with the correct grit or too long will cloud or score it inside. All this is done inside to look flat outside.
You can also paint the inside and wet sand the outside. I used...