A version using Generative Expand (*this depends on the version of PS you have):
Using the Crop Tool, crop the image down slightly inside your guide lines:
[ATTACH=full]140891[/ATTACH]
Going to the Crop Tool again, and using the Generative Expand feature (see top menu when the Crop Tool is selected)
[ATTACH=full]140892[/ATTACH]
Pull the handles out to create the empty areas to be filled:
[ATTACH=full]140893[/ATTACH]
[ATTACH=full]140895[/ATTACH]
Then click on the check mark in the top menu to fill the empty areas making sure Generative Expand is still selected
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Result:
[ATTACH=full]140898[/ATTACH]
Now crop your image to any size you want.
This is just one way to do it and again, depends on your version of PS. 