@Ronin, thanks for those suggestions. My first few checks showed "out of stock" but I can keep checking for availability elsewhere.
@TOMMI001 thanks for the news, maybe post again when the units are available? I am in the US, I'm not sure mail from your location in the Netherlands would make sense, but we'll see...
I am also considering ordering some perforated aluminum sheet such as https://www.onlinemetals.com/en/buy/aluminum-perforated-sheet which is quoting about $5 per for 10 units of "custom cut" 1x5 inch panels, or $40-50 for me to get more than enough vented blanks as I would need, which is not bad. I've seen people in other forums say the cut tolerances are not exacting SO one might need to buy big and cut down. Do ya'll know what capacity metal sheers are needed to get clean edges / angles / dimensions on blank-sized metals? I don't have any such tools presently.
-- nickgreenberg
How would you mount the perforated aluminum to your rails? There's no guarantee that any of the holes will line up or be big enough to pass the threads of a screw through.
I wouldn't use sheers as they will probably curl your aluminum. Sheers (aircraft sheers are best) only make sense for rough cuts in my opinion. I'd use a small hacksaw. You can clean up your cuts using a deburring tool. Just do a search on Amazon for one.