ultimately, a good plenum chamber is a series of panels, with multiple holes drilled in them (the holes in one panel should be offset from holes in the panel behind it).  And the panels should be a few inches in front of the fan.

The idea is to give the air a chance to slow down and even out before venting.  Why people need (or think they need) this in a spray booth is beyond me.  In a car, it makes sense; even exhaust output makes for a better tuned engine.  In a spray booth, it's really not that important.

-Fred