Not a Chinchilla
The Revelate Designs Viscacha Seat Bag (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.
Take off your rear bike rack and take the Viscacha on the Great Divide Route, the Iditarod trail, or a long day trip.
Mountain bikers with full-suspension bikes and toad bikers with your carbon frames, this is the bike packing 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 bag's 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.
Apidura Road Frame Pack
Carradice Super C Saddlepack
Revelate Designs Sweet Roll Spacer