Both the Mustang and the Camaro convertibles - what you'll most likely end up with - do not offer factory wind deflectors in any trim level.
The only wind deflectors on rental convertibles I have encountered were on Chrysler 200s (Limited model) and Audi A5s. As the once-ubiquitous Chrysler has been discontinued and phased out pretty much everywhere except some Hawaii stations like KOA which tend to keep their cars for longer than usual, your chances are indeed pretty much zero.