Zefal Z Light XS Seat Bag
This seat bag from Zefal is a compact bag that fits to your seatpost using velcro straps. The Z Light XS is lightweight at 34 grams and has the capacity for 0.3L of storage. This is the ideal option for carrying the basics such as your phone, wallet and keys.

Features
Capacity of 0.3L
Weighs 34g
Fits to seatpost & saddle rails via velcro straps
Waterproof zip
Rear opening
Rear light strap bracket so you can attach a tail light
Reflective logo