Depending on how large a bag you want back there, I use a small one (see photos below). This was my own creation, after looking through the shops I bought a tank bag and cut the side straps & magnets off. I’m not a saddle-bag guy so this works well for me, the bag is the same width as my seat. As you can see it also provides some back-rest support. 3rd photo shows how I ran a wide velcro strap under the seat to pull it down. When I want to remove the bag I simply undo the velcro and it slides off. Works great for me. One tip; if you wear a backpack then you wouldn’t want to use this bag at the same time because the backpack won’t be able to drop down to the propper resting place. I use both, just not at the same time.