Others have already offered good solutions. A custom
gobo can give you very precise results in terms of roundness and diameter from whatever angle and distance you want. Or, the easy solution (and cheap if the
gobo is already in
stock) is often to just
trim your
first electric and its
masking 12"-24" lower than the rest. If your moon will be a reasonably naturalistic (small) size, and located in the top third of your
backdrop then this arrangement tends to work fine.
I know you asked about a
gobo specifically, so maybe wandering off topic, but what about a moon box? Could be problematic in terms of resources, or if your
drop is painted or has seams in the wrong places, but I'm a big fan of making "the real thing" as in a light box that actually emits light instead of just having the
drop reflect light from a
gobo projection. It takes a little more money and labor, but is really just some plywood, white paint and some carnival
socket or something to make it glow. If you do a good job you will have a versatile tool to add to
stock. Use it a few times and it becomes cheaper than collecting gobos. Make one with a nice wide diameter to have size flexibility later. You can just swap out the facing with different shapes,
gel colors, painted plexi (for moon craters,
etc.) to make a variety of moon phases, suns, planets in various sizes. It hangs space-efficiently immediately upstage of the
drop. Some creative rigging can even allow for a rising/setting movement with the pull of a rope or two. I'm not next to my collection of
Technical Design Solutions for Theatre volumes, but I remember there is a really nice version of a moon box
in one of them that has moon
phase changing capability built in (i.e. you can have a live progression from a new moon in scene 1 to a full moon in scene 10).
): a 6x12 w/ 
