Our rack-top bag is a universal design, intended for all 4 models of our bikes.
There are 6 velcro straps to secure it to the rack on your bike: 2 at the front, 2 at the middle, and 2 at the rear.
For best results, start at the front of the bag (near the seat) and get those two straps as tight as possible with as much contact between the mating surfaces of the velcro. Do not use the front most rail-- use the second one back to start.
Then proceed to the middle 2 straps. Again, get them as tight as possible on the side rails.
Lastly, wrap the last 2 straps around the back rail of the rack. Pressing against the velcro by reaching your hand inside the bag is helpful.
The bag has more structure when there are things in it.
I hope this helps!