My cover breathes as well as without it, and looks better IMO. Trace out the pattern you want, then cut it out with a jigsaw, coping saw, dremel, mill, or whatever else you have at your disposal. Lightly sand your edges to remove burrs.
Go to Home Depot. Buy the mesh that they use to keep leaves out of raingutters. It'* aluminum. Glue and tape it to the backside of the cover (underside) with silicone RTV (not latex). PURE silicone RTV sealant/adhesive. You may want to paint before you do this.
Painting can be done easily. I used a 'cast metal' look paint, then lightly sanded my lettering to expose it in the base plastic color.