hi Kev - may i suggest that you don't actually need much in the way of venting if it is designed well. A cover elevated 20-30 inches over the trays with exhaust fan, expelling to the outside is good enough. The thing is, to have a vent located somewhere that will make the flow of air travel from the inwards vent, across the trays to the outwards vent. Putting the vent in the door is usually easiest.
To get a little techy: the size (or area across) of the inwards air vent should be the same or larger than the size of the outwards air vent.