How to use GeometryProxy outside of a ViewBuilder?

Question

I implemented a BasicContainer upon GeometryReader with the functionality of a @ViewBuilder. How can I use this inside GeometryProxy, outside when declaring the content?

Here is my BasicContainer:

struct BasicContainer<Content : View> : View {
    
    let content : Content
    
    init(@ViewBuilder content: () -> Content) {
        self.content = content()
    }
    
    var body: some View {
        GeometryReader { proxy in
            content
                .frame(width: proxy.size.width * 0.3, height: 200)
                // do some other fancy stuff ...       
        }
    }
}

Everything works fine, when I use the BasicContainer as it's meant to be:

struct ContentView: View { 
    var body: some View {
        BasicContainer {
            Text("Roberts BasicContainer")
        }
    }
}

But what, if I would like to have the GeometryProxy also outside, (like Apple implemented their GeometryReader), like so:

BasicContainer { outsideGeo in 
         Text("Width: \(outsideGeo.size.width)")
}

I guess some kind of a @Binding could make the trick, but so far I couldn't make it work. Is this even possible to realize?

Any help is very appreciated !!!