Richard
I use the topeak expandable trunk bag and the topeak rack mule, I use this combination because I like the easy on, easy off bag mounting system, just slide the bag on the rail and it locks on the bike, just push a button and slide the bag off the rail and take it with you. No velcro straps or the chance of loosing the bag. Mine has been in daily use for years, year round and it's still in great condition. I have a topeak rack mule rack that can carry panniers while the trunk bag is mounted. The bag is expandable and has side pockets and a compartment in the top.
Pete