Step 4: Rebuild the Corners
(CLICK FOR FULL-RES)
Use Content-Aware fill to replace missing bits. First make your top layer into a new file by going to Layer > Duplicate Layer and changing the Destination to Document: New. Use the Crop tool to crop and straighten it and the Quick Selection tool to select the upper right corner. Then choose Select > Modify > Expand set for 5 to 10 pixels. Select Edit > Fill and use Content-Aware to fill the corner. Conceal any telltale seams with the Spot Healing brush.