Fridge ventilation

.
Yes, I agree, on the inlet. As RMD says, some pressurisation will be of benefit. Also, the fan will be passing cooler air than at the exit so less stress on the fan.
It is not a 'big-deal' however, so if it is more convenient for mounting, place it on the exit.
You could also consider using a simple thermostat to control the fan and so reduce power consumption.

If the fans you use are 100mm dia computer fans, they can be wired in series and so run at lower speed but still move air, you don't need high flow amp dragging fans, all you need is to change the air and have it take away the elevated temp.

With one fan in each box delivering air to each box, the current draw is quite low, about 2.5 AH to 3 AH every 24 hours so can be left running all the time. No need for thermo switches etc. That small amount of current use is far less than the additional power two fridges will consume while continually trying to get rid of their own heat which is being reabsorbed in through their walls.
You have to stop/reduce the stewing in the box condition.

A lot of people say they use fridges in back of vehicles and canopies and boxes and are OK, but their fridge HAS to be running far more than they need to be purely because of the heat soak factor.
If they are running 24/7 then they use vast amounts of power and usually the frozen is, just frozen, nowhere near -18C. Perhaps they are oblivious to what is really happening.