I want to get a trunk bag for my bike. I would like to be able to do some light grocery shopping and commute to work with a change of clothes and lunch. I want it to be a bag that will clip onto the rack so it will be secure and quick and easy to get off. Does this mean I need to get the same brand bag and rack? Or do they make some bags or racks that are interchangable for clipping? I would like to spend no more than $50 for the bag.