Hannah, you don't need the apostrophe. It should be "Females only!"
Anyway good luck with the project. My experience is that aviation is one industry where females are more likely to be accepted than average - always there will be exceptions to that of course. Though attitudes across many sectors have improved enormously in the last 30 years - so you will need to judge the opinions you get in terms of the time scale to which they apply.
My opinion can be safely ignored if you like cos I'm male and old school
H