Put the side panel on a flat bit of wood with small packers underneath the panel to raise it off the wood slightly. Stretch out a square of stockings over it and use pins in the wood to keep it taught over the panel, the fact the panel is raised off the wood should help getting a good tension in the material. Next use modellers glue to put a seam of glue around where you want to stocking the stay. Let this dry, then trim off the excess stocking material. This should leave a square of material over the grill, which is still taught and will therefore act as a filter and look pretty decent. I did this with one of my rad grills which I use as an intake on one side of my case.