I am reconfiguring my operating position in anticipation of the arrival of a KPA500 and KAT500 to add to my K3. I plan on putting the K3 and KPA500 below a shelf and the KAT500 above. The K3 and KPA 500 will both have bales in use. The shelf is open to the rear. How much clearance would be needed to provide adequate ventilation? Thanks Ron W3ZV