Topeak Aero Wedge Strap Saddle Bag - Large - Black - 1.48-1.97 Litre

I recommend this product

I have one on each bike with tools and rain jacket. My first one I have had for 7 years and it is still as good as new and has taken a lot of battering. They fit very neatly under the saddle and secure well without moving around too much when riding on the hoods. There is a zip which allows you to increase or decrease the capacity but you will need it at max to fit a jacket 2 spare tubes a multi tool and maybe a small cycle lock. There is a very strong loop for mounting a rear light but lights have a tendency to bounce off so I use a cable tie.