That rack lacks side supports so you cant really fix a pannier. A racktop is the only solution.
If your bike has threaded eyelets I would throw out the clamp-on and fix a proper Blackburn style fully triangulated rear rack.
The full top-plate is to protect you from road spray if you lack fenders. If you have fenders it is unneccesary and limits your fixing points. With a few simple cross-members you can use bungie chords and also fix bags so that thery dont shift about.