The main aspect of a woman's figure that distinguishes it from a man's is in the side profile. If the bosom is noticeably larger than the waist, then one looks female. All you have to do, really, is make sure your lower chest area is padded out to match your bosom measurements, and then angle down to the hips, to create the traditional inverted triangle silhouette typical of a male. If you also have nice womanly hips, pad your shoulders well, and again, angle down to the hips. Broad shoulders and a nice "fill" effect for the lower chest should give you a masculine profile.

Also, the good ol' unisex flannel shirt can cover a multitude of "sins", and it kinda speaks of chainsaw killers, etc. Add a baseball cap with your hair roughly sticking out from under it here and there, and you'll look like a "good ol' boy" redneck ghosty fer sher! :->

Or, for a completely different approach... any chance this ghost could just happen to be a really good female impersonator?