Ideally the power module should be placed as low as you can, the orientation does not matter. The module is waterproof so getting wet is not a problem. I have seen them fitted on top of the slam panel with a piece of the rubber cushion removed to allow airflow. Some people place them near the coil, but Lumenition advise that is not ideal. You could fit it down low in front of the radiator if you wish.
Comes down to physics, hot air is less dense then cool air combined with the safe operating temperature range of the power module, -55 degrees to 85 degrees C. Exceeding the higher temperature can reduce the lifespan of the module.
When I purchased my kit circa 1990, I received this additional piece of advise from the importer/distributor. I followed this advice with the module working for the next 25 years.