Another tactic is to look for old louvered closet doors and build your own. We have a local Restore/Habitat For Humanity where I scored several of them for projects at a very nominal price, especially compared to new ones on-line.
I've also been wondering if square or rectangular steel gable vents would be sufficient...
Old shutters was an idea i considered. I'll have to look. My main issue is if they are are not already white and are instead some dark color (black, green, red, etc) then stripping and repainting them is not going to be worth the hassle (sanding shutters is awful) and piling on a bunch of white paint to finally cover it will end up skewing my results anyhow. I did look at brand new shutters and they are priced so high that by the time i buy them and build a screen from scratch i would be more than halfway to the price of a new purpose built model.
I would advise against any type of Stevenson screen made of anything metal. They are all made of wood for good reason. Because wood will not heat up the same way metal does. The thickness of the wood helps act as an insulator. Metal louvers will be made of stamped metal and that will be a fraction of the thickness of wood louvers and will heat up all the way through which will wreak havoc on your temp readings.
