Add CPT as subpage under custom page

You should be able to provide the slug of your top level page (admin.php?page=my_page) when registering your CPT(s):

register_post_type( 'matches',
    array(
        'labels' => array(
                'name' => __( 'Matches' ),
                'singular_name' => __( 'Match' )
         ),
    'public' => true,
    'has_archive' => true,
    // Add your page slug to 'show_in_menu'
    'show_in_menu' => 'admin.php?page=my_page'
    )
);