T-Bag brand bike bag. This goes on the backrest of the bike. Has adjustable back piece to fit most bikes. Comes with a rain cover (see photo in comments.). Multiple pockets and expandable. Zippers to keep items in and lots of ways to lock it in place with the clips. This is backpack style so can be carried around. I used this on a west coast trip as my carry on and only had this for the trip. Fantastic item. Photos make it look more faded than it is. $60.