I think a better idea is to REMOVE your top case, and in it's place get a waterproof bag from Cabelas --the biggest is too big, so choose carefully. My wife freaked at the idea-at first, but finds the large waterproof bag just as secure &more comfortable. After a lot of trips-solo and with spouse, this is how I pack:stuff for the bike(tools lubricate,battery charger,etc) and my clothes in right pannier. Wife's stuff left pannier. Cabelas. Bag everything that's left if it dosent fit into my very large tank bag ( with right and left panniers) (Touratech )And yes, I still pack too much!!
~Things work out best for those that make the best of the way things work out~