As far as i know you cannot do this.
When you add a CustomView in the Designer, the dimensions are those given by the Base object, but you cannot add any other view into it, only internally
What exactly do you want to do with this kind of CustomView ?
When i develop CustomViews i add all views insides the CustomViews code, no layout, but that is just how i do it, i do not want to depend on any layout file.