Scallops!

Here is the background scallop to place your projects on.... This will save you a lot of trouble.  There are both vertical and horizontal versions.... Remember not to skew your image to fit the scallop, but instead skew the scallop slightly to make it even all around.

Click on the stamp to expand it out to the full size then right click on it, copy (or save as) and then file new in Photoshop, edit paste....