If you are trying to find somewhere to house a water bottle and carry your jacket etc on your Swift you may like the seat bag by Axiom. It's not the biggest bag but it will hold much of what you will need for a day ride - including a bottle.

The bag sits neatly behind the seat (using Velcro straps) and suits the Swift as it hugs the long seat tube. I use a 750ml bottle which is held securely in the mesh pocket. There is a tab from which to hang a rear light but it is not really usable with the bottle in place! A standard 500ml bottle should work OK.

The main compartment holds my waterproof jacket and lightweight cycle lock and the side pockets are stuffed with two inner tubes, puncture outfit, tyre levers and multi tool. There is not much room for anything else. It looks neat and solves the bottle carrying issue of the Swift.