May also try honeycomb core for flow straightener, if can source cheap enough.
first google link (
http://www.plascore.com/honeycomb/default.asp) - comes in many sizes/materials.
At least two wind tunnels at my Univ. use some type of honeycomb material, if I had to guess about 2-3" thick.
I would be worried that light-fixture gratings wouldn't be thick enough, but not really sure of your goals / tolerances with this project.