Content Aware Fill, clone stamping, patch tool, spot healing at the edges of image

I've always had issues trying to remove or cover up elements at the edge of an image. I'm sure there are 100 ways to approach this, but I tend to seem to have issues no matter what I do. 

As an example - I have film photograph that I want to turn digital. I took a shot of it using a digital camera. I brought the digital image into photoshop to crop out the edges of the image that don't contain the film photo. When I crop around the film photograph it is not perfectly straight so there are some bits of background at the edges. I want to fill this corners in. With the part I want to remove being right at the edge I get issues no matter what tool I use and how I approach replacing those slivers of background. Is there an easy way to do this that I'm missing?