I would lay it out full size on the plywood myself. having built a few arcade cabinets in my time, that is how I did it.
for the arc, 2 options: since it isn't critical (it's just a body cutout space), take a lid from something approximate and just lay it on there until it looks good. can lid or something.
or, what I do, is take a thin 3/5" strip of flexible Masonite, drill a hole in one end, lace a piece of string through the hole, then tie it off on the other end making an arc. just slide the string until you bend the Masonite to get the curve that hits the two points.
works like a charm...
and show us the game when done, what software you using (MAME? what front end? need ROM's?), Happ controls? using a multi-ac for them or keyboard hack? trackball?
gotta see it!