I have the waterproof Nashbar variant. I don't know what the attachment system on the ones you linked are, but mine have a full plastic back, and attach by hanging hooks over a rack bar and holding on with a bungee strap low. I don't particularly like the system, if you overload the bag and get on rough roads you can easily have it bounce off, but it worked for a week tour for me and is going back on the bike for the next one (probably with some modification to keep it in place better).