ORIGINAL: MinnFlyer

The best way to do it is to tape strips of cardboard (or other stiff material) to the sides of the fuselage and cut holes in them for the head, needle valve, Muffler, and anything else that you want access to. Make SURE that the cardboard strips are secured back on the fuse FAR ENOUGH so the cowl ca slip between the cardboard and the fuse.

Then remove the engine, slide the Cowl in place (Measuring the front opening to the firewall so the prop clears properly) and use the holes in the cardboard to mark the openings you need to cut.