The rich panel is just a div from the html layout point of view. The content of the panel is a div included into the main panel div. If panel has a header, it is also a div above the content div. Therefore, the structure of the component layout is the following:
<div> <div>header</div> <div>content</div> </div>
Abstracting from the JSF, try to style the structure above like you want. If you can, that you can do the same with rich:panel, if not, we are sorry. It is just a standard html. No any magic from our side.