DIY Perforated metal grills?

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.

Go to McMaster-Carr and search for 'perforated.' You'll get lots of choices at reasonable prices.

something like this ?

http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=260-394