How to add an overlay behind a modal view using UIModalPresentationCustom

Question

I was UIModalPresentationPageSheet to present a modal in my application. I needed a bit more customization, so I moved over to using UIModalPresentationCustom. This style is very bare bones as to be expected, but what I'm wondering is if there is an easy to add an overlay behind the modal window like with UIModalPresentationPageSheet that you get for free? Is there a property or method I can latch onto or do I need to add another UIView to the container view?