Poke hole in UI Image?

I'm trying to poke holes and stuff through a UI image so the player can see the background. I thought I could do this with a mask, but it seems I need to reverse how masks work.

My image is a scroll rect so the hole can move up and down, and the player should see the background through it.

Any help? I tried a shader I found in some threads, but they weren't working in Unity 2017.

Please guys this is for a jam, I don't have much time :O

EDIT, how the background is an image too.

 

In a shader... Multiply a black, programmatically drawn, circle onto your images alpha positioned in screen uv space. Pass the coords from a script.

 

Thanks for your help, brownboot! The thing is I've 0 shader knowledge, though I'd love to learn, I'm a little tight on time. I'd really appreciate it so much if you could provide an example or something

 

It seems I forgot to mention that it needs to be a certain sprite shape. Not just a circle (Like a tear or something). Please help? About < 5 days left.

 

Pleeease help?