I have the standard top box and panniers. Couldn't stomach the price of the official bags so bought some Karrimor dry bags from the Sports Direct sale. I bought 1 light blue 25l, 3 green 15l and 1 orange 5l.
Used the 25 and a 15 in the LH pannier, two 15's in the RH pannier and the little one for must keep dry valuables in the top box.
You could easily get 2 15l in the top box and are very good quality and waterproof.
In sale total cost of all five about £17.
Had some quite heavy rain on NC500 in June and the boxes didn't leak anyway but maybe because new and seals still fresh.