Better yet, just run the fan all the time. It really helps with keeping shanks cold and reducing stratification, or scrubbing heat from an active ferm. The freezer shouldn't turn on that often, so you won't get much benefit from the fan only being on when the freezer is on. Also, the evap lines in the keezer are still dropping in temp for a few minutes after the freezer shuts off. An always on fan helps spread that around.
A small 12V fan from a computer will run on most ~5V cell phone chargers, and will provide more than enough circulation. Total power would be ~1W, or less.