Timbuk2 uses their experience creating their highly regarded messenger bags to make these very durable and functional seat bags. The medium size can hold 1 mountain or 2 road tubes, patch kit, mini tool, tire levers keys etc. while the large has plenty of room to hold 2+ mountain tubes.
The Seat bags are made with ultra durable ballistic Nylon with rugged zippers. They are held in place with Velcro straps that wrap around each individual saddle rail and around the seat post. There is even a key ring tether inside which makes locating the keys after a ride super easy.