I was intending to put a luggage grill on the top box to hold light waterproofs so that I can get to them quickly. However, I saw a TTSE with GIVI bags either side of the rear seat on top of the panniers, secured with ratchet straps through the pillion grab rails. The panniers could be opened / closed / locked / unlocked with the Givis in place and they did not interfere with the pillion's comfort or safety. No need to drill holes in the lid of the top box. The Givi were sausage shaped with lashing points. I think they were Givi Silver Traipack -T477 Roll Bag but don't quote me.
Freeway Phil