Another more cost effective than bike manufacturer's products are Kathys Bags (no remarks please

)
I have them and they fit the RT panniers precisely with lots of nice touches - the blurb says:
"Each big compartment opens around three sides for easy access with its own 2-way zip. There is a an external zipped pocket on one side which, when face-up in the top case, is readily accessible when the lid is open"
but they also have an inside zipped pocket. I can't fault the JS bag solution for price but Kathys Bags make life so much easier. Really well made, and in a lovely part of the world too.
Get them here:
http://www.chainspeed.co.uk/brands/KATHYS.html