I've used the large Topeak top fuel bag on my Radrover (eBay for about 15-20% cheaper sometimes). I had this bag for +3 years and I transferred it from my other pedal commuter bike. I used it to hold my riding glasses, iphone 4S cell phone, wallet, and headlight battery. Added a cell phone holder for tracking exercise apps when riding and upgraded my lights with a battery too large to fit in the bag. I already had a Topeak MTX DXP rack bag and the top fuel bag wasn't being used as much and I took it off.
RadRover with large top fuel bag: (Link Removed - No Longer Exists)