Как расширить Box до внешних границ в Java

public class SimpleWindow extends JFrame {
    SimpleWindow(){
    //Main Window
    super("Testing");
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setSize(900, 500);

    // MAIN JTabbedPane
    JTabbedPane tabbedPane = new JTabbedPane(JTabbedPane.TOP, JTabbedPane.WRAP_TAB_LAYOUT);

        //TEST TAB
        JPanel testtab = new JPanel();
        tabbedPane.add("testtab", testtab);
        //Test Box
        Box testbox = Box.createHorizontalBox();
        testbox.setBorder(new TitledBorder("Settings"));
        JComboBox main_status_levelsa = new JComboBox(elementsa);
        main_status_levelsa.setSelectedIndex(0);
        testbox.add(main_status_levelsa);
        testbox.add(new JCheckBox("Use present time."));     

        testtab.add(testbox, BorderLayout.CENTER);