Only drawback to the crop cut/paste attributes approach is if you happen to reposition or scale even slightly one clip you’ll notice the border shifting as well on that edit.
Instead I would recommend creating a custom matte. It’s better if you have Motion, otherwise start with the Shape generator in FCPX. Turn off fill and adjust the size of the outline and then the x-y scale in the Video tab and you should be able to get the right shape. That will then go as a connected clip over the entire edit and you can still make crop and scale adjustments to the clips underneath.