I've tried a plethora of scribers, picks etc. over the years. I still have not found a tool that does the curves of a door or trunk line great.
I think I'll give the thread method a try some day.
I'd also like to see a tool that would allow routing a channel for a window to set into so glass could be installed from the outside like a 1:1.
Try the straight pins for doing the curves, hold the pin close to the pointed end for more control.
You could try making a tool with the description I made above, just leave out the pointed edge on the right. File the flat edge at an angle so it could scrape the plastic. I don't know for sure if it would work, but it's worth a try.