The Boron is much more forgiving in terms of Input Voltage.
As said previously, you can use your (6V) 9W panel directly with a Boron or Electron (but Not Photon, Xenon, Argon).
Or something cheaper like a standard 12V panel, along with a 12V SLA Battery if space isn't a concern.
Another benefit to having a 12V battery with much more capacity than you need, is that it allows you to easily add a timer relay to minimize the chance that you will need to travel to the site to Reset the Boron in the future.