Take off your rear bike rack and take the Viscacha on the Great Divide Route, the Iditarod trail, or a long day trip.
The Viscacha (pronounced vis-KA-cha) is named after a rabbit-like critter that is not actually a rabbit. It follows that the Viscacha Seat Bag bears a passing resemblance to a seat bag, but with a 854-cubic-inch capacity, a roll-down opening, a removable internal compression strap to aid in packing and closure, and a wide seat post gripper wrap attachments for extreme stability.
Mountain bikers with full-suspension bikes and toad bikers with your carbon frames, this is the bikepacking seat bag you've been waiting for. High density foam stiffens up the seat post connection and a flexible plastic bottom sheet adds support along the bags length.
You'll need a minimum eight inches clearance from your saddle rails to top of tire, and a minimum five inches of exposed seat post. If your bike doesn't meet those minimums, the Revelate Designs Pika Seat Bag fits bikes with less room to spare (and it's named after a critter even smaller than the viscacha).
Note: If you run your saddle all the way foreword on the rails the bag will rise above the saddle in the back due to the sharper angle between the seat post and saddle attachment.
Revelate Designs Ranger Frame Bag
Thule Sport Rack
Thule Tour Rack