TOPEAK MTX Trunk Bag for Bicycles - Waterproof Bike Rear Rack Bag with Expandable Storage - Perfect for Commuting, Touring & Outdoor Adventures

The pack slides into the TOPEAK rack and locks securely to the rack's front rail. There is a loop on the back of the pack to hang my tail light. The main compartment of the pack is roomy enough to carry my shoes, towel, etc. to my workout. I haven't used them yet, but the sides open and drop down to give even more storage for the longer rides I have planned.The only downside that I can see is that the pack is very easily removed from the bike. This is very good for me to carry the bag with me but not very secure if it is left on the bike unattended.