Why not look for some perforated sheet metal, cut to size, paint black to loosely match the far side of the case. A very quick look around suggests you can get stainless steel sheets 50cm x 20cm with 3mm holes for about £20 on the bay. Other sources will be available with more looking.
edit:
diy-mesh.com
seems to have hexagonal hole metal which would match the white of the case too.