Just create a shell account and let people start creating outfits. The more work they do, the more likely they are to want to “Save”, i.e. create an account.

I wonder if you have stats on how many sessions click “Closet”, see the signup page and don’t click through.

Let prospective users actually do something with your app, and as long as the session is unregistered keep an obvious button to Save/Register on screen.