For Easy, DIY, I would buy a buck converter module and put it between your power bank and your flashlight.
If you search e-bay for '3a buck step down', you'll see some little adjustable switching regulators. If you need more current, then just search 'DC DC Buck' and order by price. The small ones a rated 3A, but they will likely do maybe 1.5A without cooling of some kind. Simple and easy to use. Just make sure you use a multimeter to adjust it to the voltage level you want. 3.7 volts and then hot glue the pot so it doesn't move.
As mentioned, you will need to know whether your USB power bank will be able to handle the current. You'll need to know how much current that flashlight uses. Some power banks will put out quite a bit of current. These aren't PC USB connections and many are rated for 2.1A and some will do more.