- Find a pair of vehicle radiators that are as close to a box fan in size as possible.
- Zip tie them to either side of the box fan. As the fan blows: it will draw air in through the “second radiator” and blow it out through the “first radiator”.
- Hook the out of the first radiator to the in of the second using flexible hoses. Cheap garden hoses might even fit.
- Hook other hoses to the in of the first radiator and the out of the second.
- Run water on through the first radiator, out of the second. This makes the most efficient heat transfer possible.