At a whopping 16L, this is the biggest bag you can fit under your seat without a rack. For a smaller version, check out the Dropper Seatpack. 
It is meant for light, large volume items like a sleeping bag, tarp, and clothes. This bag feels solid even loaded up on the bumpy stuff because it has a 360° internal plastic frame that holds its shape. Rugged metal cam buckles means secure, no-slip connection points. Durable, water resistant construction. Good thigh clearance with room to get behind the saddle. This is a no-frills work horse with one strap for compression and extra gear.
*Must have 4.5" of exposed seatpost and 8" saddle-to-tire clearance
 
Features
- 
16L max. volume // Weight: 21.8 oz 
- 
Specially shaped 360° semi-rigid plastic frame 
- 
Rubberized contact patches under saddle and around seatpost 
- 
6-point webbing attachments 
- 
Corrosion resistant metal-cam buckles 
- 
Roll closure with velcro 
- 
Compression strap can be used to strap on extra gear 
- 
Minimal seam construction for water resistance and durability (the top and sides are one piece of fabric) 
- 
PU coated Cordura (water resistant) or X-Pac main fabric +$15 (weatherproof) 
- 
HANDMADE IN CALIFORNIA 
 
Instructions
- 
Lace webbing straps as shown in photo below 
- 
Attach velcro strap(s) to seatpost 
- 
Tighten webbing straps evenly to center and stabilize the bag