How to cut a hole in a view using a mask

I need to make an inverted mask. This means making a "hole" in the rendered view, instead of rendering inside the hole. Since this plugin uses alpha to define the mask, I cannot use a transparent view inside an opaque view, otherwise it will only use the opaque view as the mask. It would be better to use white instead of alpha to change the mask opacity. Is there a way to do this? Here's an example of what I want to do:


I want to remove the bottom part of that rectangle, using another view with rounded corners. But I'm unable to "remove" a piece of the mask. Is there any workaround?