Best way to add a sump to AIO

[JReefaddict]

So I've been cooking up some ideas lately and I was wondering what the best way would be to add a sump to an AIO tank. I'm going to be changing the sump on one of my tanks and want to be able to add a 20g frag tank as a second display tank and I liked the JBJ 20g shallow rimless tank which is an AIO