Ivor, you can use the in_category($cat) template tag, to determine if the post is on the specific category. $cat can be either the category slug, the category name, or the category ID.
So, you can use the template tag to determine if the post being edited (since the post being created doesn’t have a category yet), has the specific category. This way you can conditionally include the post metabox, based on the category of your preference.
For example:
if ( in_category('Design') ) {
add_meta_box( ... args ... );
}

