A small rear-mount specific bag mounts to your top tube and your seatpost. The Jerrycan® ekes out even more carrying space right on the frame of the bike for spare tubes, tools. or some food
It's only two inches wide, so it stays out of the way of your thighs.
There are two Jerrycan models:
Regular - For most bikes
The seat post / top tube angle is slightly obtuse to fit many frames. If your bike has an acute angle there (really sloping dropped top tube) then it will just be pushed foreword a bit and take up more space along the top tube.
Bent - For bikes with gusseted braces at the seat tube/ top tube junction.
Surly Nice Rear Rack
Surly Nice Front Rack
Revelate Designs Terrapin Seat Holster