What is a user-friendly approach to exiting a preview screen?

I have a mobile Android application that allows users to specify through a form the layout of elements displayed to the screen. The user can also preview the customized layout in full-screen. However, use of the back button (soft key) is unavailable, and a navigational header adds clutter. Is there a clean, intuitive approach to respond to user input in order to exit the preview?

I have considered adding a button as well as tap to exit approach. A button is not very desirable because it adds clutter. In addition, the button may be assumed to be part of the preview. Allowing the user to tap to exit is okay, but I find the approach to be susceptible to accidental touches and not immediately intuitive to a user. Responding to multiple touches in a period of time is not a terrible approach; i.e. double tap.