based on https://www.youtube.com/watch?v=KNDZZ5bD-wM
I would not recommend it. The rear radiator is apparently only element doing some cooling. Inside, there is no radiator at all. Fan's can exchange air as much as they want, but if there is no radiator inside, which would cool the chip, internal airflow might be not enough.
In the end, it will really depend on airflow around the rear radiator. If you put it into small sealed box, it will overheat and it is not going to matter if it is horizontal or vertical.
Radiator is clearly designed for horizontal operation due to shape and angle of ribs in order to make airflow of hot air (coming up) easier.