How can you project a line onto a surface?

I want to create a monocoque structure for my aircraft but it seems I can simply constrain the sketches to the inside of my fuselage skin.
How would I go about creating a line around the skin interior so I can potentially multi-surface the structure?
OR if there's another better way you guys can think of doing the monocoque structure could you refer me to a video or something?
P.s. I'm not allowed to do a truss/tube chassis