Boris, Get a profile gauge. Go to this link to see a picture and description of the tool: https://en.wikipedia.org/wiki/Profile_gauge. You push the gauge into the fuselage half following the line of new bulkhead you wish to install. Then lay the gauge on a folded sheet sheet of paper, using the fold as the center line, trace the rough outline. Cut out and unfold and you should have a full pattern. If fuselage is not symmetrical, use gauge on both halves and join shapes for single pattern. I have use this technique on both injected and vacuform kits.

I use thin cardboard or printer paper to form a template. You can usually eyeball it pretty close, or fold it into the fuselage to get a basic shape, outline the shape with a pencil, then trim it to fit as needed. If you make a mistake, just get a fresh piece of paper and start over. Trace the outline onto your sheet plastic and you're good to go. Not very scientific, but it works pretty well and you don't have to buy anything!