So you'd only want to draw in lines for these spots here, the other 2 spots are slightly curved and you want to use the existing edges there rather than putting in a straight line:



- Michael

Hi Involute, so in the attached file I ended up using Edit > Trim to cut pieces away and then used Construct > Network to build a filling surface and joined it in.

Also instead of just drawing in a line for the top of that red zone above I put in a control point curve that's slightly curved so it's smooth with the lower piece.

- Michael