Just one thing to be a little bit easy and a little bit quick?
Do you still want to look your absolute best while wearing a cape? Do you want to put much effort into your costume? Not really, because you're in it for some kicks but you're not super serious about being canonically accurate about the whole Supergirl persona. Except you're a girl, and you're not dressing up as Superman if you don't have to. Of course, who doesn't want to show off a little? And what is their costume, you're wondering? And so you need a costume.Well guess what, lady? So Batman needs a good teammate, again which means that you're going to need to pull off a convincing Superman on short notice.