When I've built speaker cabinets I made wooden frames, painted them and then wrapped the grill cloth around the frame and stapled it on the backside. To keep them in place I used a bit of Velcro on the corners.
If your metal grills are the diamond shaped type screwed to small wooden blocks on the baffle you might try attaching a wooden frame on the back of the grill and use that to staple the cloth to. You could then use the frame to attach the grill to the baffle.
Keith