Scribe tip; make a two-part top-hat profile plug that fits that center hole (just round, nothing fancy). Upper top-hat fits the hole diameter, lower top hat is a little bigger and the right length to keep the whole thing steady or back-screw to a scrap of masonite or plywood. Next; run a small bolt & nut (or threaded rod) thru the center of plug-set (say M3), run a 1/2" x 1/16" brass strip off that center. Slightly form the brass to the saucer profile (not all too critical for diameter scribes) and tap & drill (7) holes in the strip at the intervals you want the diameters "scribed-in". Depending on the scribe tip you select, you can even put a nut on the back of it (in my case I use a scribe tool with a replaceable carbon tip and disassemble-it/ reassemble-it at each radii hole. For the radial lines you sorta do the same except you need very thin "P" shaped brass arm that closely follows the hull contour. The "P" shape allows you to line-up the scribes with the true center of the threaded-rod/bolt.
Keep the rig. I have mine hung on a wall like a picture and have several plugs and arms made at this point....