I'm not sure if there is a way to limit the depth of Flat View, without losing the way it is a "live" view of the folders (i.e. updates automatically when things are added/removed). You can apply real-time filters to Flat View but only based on file & folder names. (Unless there's a trick I can't think of right now.)
If you don't need a live view, and a snapshot of the files/folders would do, then it's something you can do fairly easily using the find tool with a sub-folder filter.