Ok. This involves the same components as an oil-system of a engine, basically. What you need is a pump and a cooler, thats the basis. Then you need to make a two holes in the casing, one for taking warm oil out of the box, and one for letting the cooled oil return from the oil-cooler into the box. The hole made for taking the warm oil should be located somewhere where its guaranteed to be oil all the time, i.e. the bottom of the gearbox. The return hole should be located so that the cooled oil mixes with the oil already in the box before it comes in contact with the gears and other mechanical parts of the box.

Hope this sorts it out for you!