Hardware stores carry perforated metal, it is a bit of a bear to cut but I imagine it'll get the job done. If you need larger sizes you may consider talking to Martin Logan and buying some material from them, they use large perforated grills on their speakers.
DIY Perforated metal grills?
In the 80's my father had a pair of ADS speakers with a matte black perforated metal grill and I always loved that look, can material be purchased already perfed and then cut to size, painted and mounts attached?..(paint and mounts DIY)
thanks
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