Give a unique ID while saving

When you add a meta box, the post object is passed to the function that outputs your meta box content, and the ID is immediately available. Here’s a quick proof-of-concept that will put the ID in a field in a post meta box when creating a new post:

function wpd_sku_meta_box() {
    add_meta_box(
        'wpd_sku',
        'SKU',
        'wpd_render_meta_box',
        'post'
    );
}
add_action( 'add_meta_boxes', 'wpd_sku_meta_box' );

function wpd_render_meta_box( $post ) {
    // $post->ID contains the ID of newly created post:
    echo '<input type="text" id="wpd_sku" name="wpd_sku" value="' . $post->ID . '">';
}

function wpd_sku_save( $post_id ) {
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
        return $post_id;

    if ( ! isset( $_POST['wpd_sku'] ) )
        return $post_id;

    $sku = $_POST['wpd_sku'];
    update_post_meta( $post_id, '_wpd_sku', $sku );
}
add_action( 'save_post', 'wpd_sku_save' );

but please don’t use this code as-is, start with the example on the add_meta_box codex page, which has some nonce and permission checks.