I've cut mine down to the tops of the sides' PF logos, then in the middle for 12cm I've done a further step down of about 6-8mm. Gives enough downforce, keeps good speed - I'm also keeping that shape for upcoming #2.
What, the fill-in bits (like the green above) or the actual grooved lines on the shell?
I use a medium-sharp Sharpie pen on the outside rubber frames.
I don't usually bother with panel lines on a runner, but the thinnest-sharp Sharpie does it well.
For the green... I pulled off Proline's vinyl masking, the inner lines were visible. So I cut out the 'windows' with scissors & reapplied them + shot the met green. Then removed and shot with PS31 Smoke to tint.
(I didn't throw away the used masks either, hoping I can reuse for upcoming #2...