Even though that article is for v7, the idea is still valid. Currently there is no webpart for a modal window mainly because it requires 2 different html elements, usually in 2 totally different and very specific locations. So not the easiest thing to do.
What we typically do is create a button with a JavaScript click event handler. Place that button in an editable text area or a static text area. Then add a static text web part to the template which contains your modal-dialog panel and content.