Basic Border is obviously the first choice, but it doesn't work if you're using crop tools on the picture-in-picture (eg. FCP's own Crop, Mask Shape etc.). However, if you use Basic 3D, Digital Heaven's DH_Box or Mask Shape to resize the image, you can then copy those filters to a white Color Matte and then adjust the size to slightly larger than the image on top. It'd be faster than repositioning the white shape by hand.
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