Hardware cloth stainless steel mesh – Home Depot, Lowes, True Value, etc. It's a pretty common item sold in hardware stores. You can look online too if you can't find mesh that is small enough. Be sure it's galvanized stainless steel and smaller than 1/4" (but not so small that air has trouble going through something 'solid' – something like 4-5 mm seems sufficient).

Something like this maybe: https://www.amazon.com/Activists-Woven-4-5mm-Coarse-Stainless/dp/B07SXGTP2G/

I'd look in store b/c it tends to be far far cheaper.