This is a handy container that allows you to specify a width proportion for its subviews

ProportionalZStack.swift

import SwiftUI

/// A view container that overlays its subviews with a given width proportion, aligning them in both axes.
///
/// The `ProportionalZStack` uses a width to specify how much of width proportion its subviews must take.
///
/// The following example creates a `ProportionalZStack` with a proposed width of 70%,
/// an aspectRatio of 4:3 and an offset of 80% for its subviews.
///
/// ```
/// ProportionalZStack(width: 0.7, aspectRatio: 4/3, y: 0.8) {
///     RoundedRectangle(cornerRadius: 15)
///         .fill(.red)
///
///     Text("Hello, World!")
///         .foregroundColor(.white)
/// }
/// ```
struct ProportionalZStack: Layout {
    let width: CGFloat
    var aspectRatio: CGFloat?
    var x: CGFloat
    var y: CGFloat
    
    /// Creates a new ProportionalZStack container
    /// - Parameters:
    ///   - width: The proposed width you want for the subviews, 0.0 = 0% and 1.0 = 100% width.
    ///   - aspectRatio: The ratio of width to height to use for the resulting view.
    ///     Use `nil` to get a flexible height for the resulting view.
    ///   - x: The offset `X` for the subviews, 0.0 = 0% and 1.0 = 100% Y. The default value is `0.5` meaning `midX`.
    ///   - y: The offset `Y` for the subviews, 0.0 = 0% and 1.0 = 100% X. The default value is `0.5` meaning `midY`.
    init(width: CGFloat, aspectRatio: CGFloat? = nil, x: CGFloat = 0.5, y: CGFloat = 0.5) {
        self.width = width
        self.aspectRatio = aspectRatio
        self.x = x
        self.y = y
    }
    
    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGSize {
        proposal.replacingUnspecifiedDimensions()
    }
    
    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) {
        for subview in subviews {
            let subviewWidth = max(10, bounds.width * width)
            let subviewHeight = aspectRatio == nil ? bounds.height : subviewWidth / max(0.01, abs(aspectRatio!))
            let subviewProposal = ProposedViewSize(width: subviewWidth, height: subviewHeight)
            let xPosition = bounds.minX + bounds.width * x
            let yPosition = bounds.minY + bounds.height * y
            subview.place(at: CGPoint(x: xPosition, y: yPosition), anchor: .center, proposal: subviewProposal)
        }
    }
}