如何在自定义贴子类型的自定义metabox的WordPress中添加滤镜？

// 注册自定义贴子类型
function custom_post_type() {
    $args = array(
        'public' => true,
        'label'  => 'Custom Post Type',
        // 添加其他参数
    );
    register_post_type( 'custom_post', $args );
}
add_action( 'init', 'custom_post_type' );

// 添加metabox
function custom_metabox() {
    add_meta_box( 'custom_metabox', 'Custom Metabox', 'custom_metabox_callback', 'custom_post', 'normal', 'high' );
}
add_action( 'add_meta_boxes', 'custom_metabox' );

// metabox回调函数
function custom_metabox_callback( $post ) {
    // 添加metabox内容
}

function custom_metabox_callback( $post ) {
    $selected_filter = get_post_meta( $post->ID, 'selected_filter', true );
    ?>
    <label for="selected_filter">选择滤镜：</label>
    <select name="selected_filter" id="selected_filter">
        <option value="filter1" <?php selected( $selected_filter, 'filter1' ); ?>>滤镜1</option>
        <option value="filter2" <?php selected( $selected_filter, 'filter2' ); ?>>滤镜2</option>
        <option value="filter3" <?php selected( $selected_filter, 'filter3' ); ?>>滤镜3</option>
    </select>
    <?php
}

function save_custom_metabox( $post_id ) {
    if ( isset( $_POST['selected_filter'] ) ) {
        update_post_meta( $post_id, 'selected_filter', sanitize_text_field( $_POST['selected_filter'] ) );
    }
}
add_action( 'save_post', 'save_custom_metabox' );