I added a little .css file " .config/gtk-3.0/gtk.css" copied below. (there's actually a couple approaches I took, the one I'm using here is not commented out).

/* Two different approaches given below
   both valid but with slightly different
   behaviour
*/

/* This first approach aggressively radiuses
   everything, even items within the panels
   themselves.
*/

/*.xfce4-panel {
   border-bottom-left-radius: 16px;
   border-bottom-right-radius: 16px;
   border-top-left-radius: 16px;
   border-top-right-radius: 16px;
} */

/* This approach is not as aggresive as above.
   Will need to add some transparent seperators
   on either end for the radius to show.
   (16 px for full radius at my current settings)
*/

.xfce4-panel#XfcePanelWindow {
   border-radius: 16px;
}
@import 'colors.css';