Cutout to background layer

I'm looking to make an effect where I only render a tiny piece of a large 2d background, like a cutout of it. I want to be able to move around this cutout, and see only the piece of the background below the cutout. There will be other art in the scene as well but that should be irrelevant. I'm drawing a blank on how to go about this, any ideas? Thanks in advance.

 

If the background is on the canvas (I think it only works there) you can try mask or mask2d.