They might be cheap, but I have never seen a box fan fail, and I have a few in service for many years. Consider that they are designed for cooling in a window

They might be cheap, but I have never seen a box fan fail, and I have a few in service for many years. Consider that they are designed for cooling in a window application, which means they must be able to survive operation 35C in direct sunlight. As an air filter, mine operates at 22C in shade. That easily makes up for the temperature differential due to load conditions.