Poke hole in UI Image with a mask?

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.

bump

bump! Please guys this is for a jam, I don’t have much time :open_mouth: