The Mac Mini bottom part contains a fan that draws the hot air from the circuits and, apparently, just projects that hot air on the Mac Mini bottom cover. However, there is no proper exhaust on the bottom cover (no holes for the heat to escape through). To lower the temperature of a Mac Mini by about 20C / 68F, the bottom cover can be perforated and a grill installed.

It would be nice if the bottom cover could be replaced entirely with a metal grill that would allow the heat to escape. However, even with a few perforated holes, the temperature drops considerably.