Working on the wind-tunnel, I noticed that wind blows outward from the fan only directly in front of the fan blades. There is an inward flow from those areas not directly in front of the blades. I created a new cowling which wraps around the perimeter of the fan to cut off flow from those areas. The result is a speed increase in the chamber from 3.6 m/s to 4.1 m/s, a 14% improvement!
In mapping the flow field in chamber it turns out that the boundary layer develops quickly with distance from the front edge. This chamber will be barely large enough for the 30cm x 30cm plate; its boundary layer will impinge on the leeward corners of the plate.