GridBagLayout panels alignment

Question

I have a little issue with the GridBag Layout Manager. I am trying to display 9 panels like this:

Answer 1

The accepted answer is appealing, but it causes section 4 to widen faster than the flanking panels as the frame is resized. This variation places panels 8 & 9 in a separate sub-panel of a BoxLayout.

import java.awt.*;
import javax.swing.*;

/** @see https://stackoverflow.com/q/14755487/261156 */
public class GridBagDemo implements Runnable {

    private JPanel panel;

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new GridBagDemo());
    }

    @Override
    public void run() {
        JFrame frame = new JFrame("GridBag");
        frame.setLayout(new BoxLayout(frame.getContentPane(), BoxLayout.Y_AXIS));
        panel = new JPanel(new GridBagLayout());
        add(1, 0, 0, 1, 2);
        add(2, 1, 0, 1, 1);
        add(3, 1, 1, 1, 1);
        add(4, 2, 0, 1, 2);
        add(5, 3, 0, 1, 1);
        add(6, 3, 1, 1, 1);
        add(7, 4, 0, 1, 2);
        frame.add(panel);
        panel = new JPanel(new GridBagLayout());
        add(8, 0, 0, 1, 1);
        add(9, 1, 0, 1, 1);
        frame.add(panel);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    private void add(int i, int x, int y, int w, int h) {
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.gridx = x;
        gbc.gridy = y;
        gbc.gridwidth = w;
        gbc.gridheight = h;
        gbc.weightx = .1;
        gbc.weighty = .1;
        gbc.anchor = GridBagConstraints.CENTER;
        gbc.fill = GridBagConstraints.BOTH;
        JPanel p = new JPanel();
        p.setBorder(BorderFactory.createEmptyBorder(8, 8, 8, 8));
        p.add(new JLabel("Panel " + i));
        p.setBackground(Color.getHSBColor((i - 1) / 9f, 0.75f, 0.95f));
        panel.add(p, gbc);
    }
}