I've never had the need to make my own parts, but there are several times that I have mended plastic parts with a hot air welder.
Take this guy here.
That's the ABS plastic panel that holds the entire heater control unit in the dash of my Mercedes. The part is unavailable without purchasing an entire used heater control unit, which at the time couldn't be touched on e-Bay for less than $800, and every junkyard donor I found was either missing the part I needed, or it was completely shattered.
I fixed it myself using a white ABS welding rod. (Colour doesn't matter since a wood panel hides the plastic plate once it's installed.) Once the panel was back together I ground it down to original dimensions so that the wood trim would sit flush on it and re-drilled the mounting holes. It works perfectly.
My cheap hot air welder is probably one of my better investments. It takes some practise to be able to make a good weld without destroying the parts you're trying to repair, but once you get the hang of it, you can do some pretty amazing things.