Bit late with the reply and only an amateur but I use greaseproof, a light box and I draw a straight line down the middle and tape my greaseproof to the line. Put the shell over the light box and proceed to trace the design making sure that the paper contours the shell nicely.
I also will mark say the body pin dimple front and rear and mark the top and bottom of the front window so when I put the paper inside the shell to do the relief tracing I also tape the centre perfectly to the line and have the dimple points and front window as extra references.
Still perfecting this as I'm sure you can tell from my painting but thats how I does it.
