I have used a 12v
surplus computer "muffin" fan with a resistor in series so that it runs on about 6 volts. Put it anywhere that will increase the airflow over the finned radiator on the back of the refrigerator. I have also used 2 small surplus fans wired in series. The fans run slow, but only a whisper of air is needed to improve cooling.
Camping world, et al, have plenty of fans for less than 420 with thermostats so that they run only when needed. Some are even solar powered.
On another thread, there was a part number for a thermostat that will turn the fans on onlywhen the compartment gets hot.
Some folks are installing expensive fans, but, IMHO, that is overkill. A regular fan will be silent when run on part voltage and will move plenty of air. After all, the refrigerator was designed to operate with just thermal air movement; any increase in air movement is a bonus.