Here is what I used, which is made for exactly what you are using it for. It is made of steel mesh and covered with a cardboard like material. Got it at my local NAPA dealer and it was only $13.00. Worth a try if you have a way to sketch a pattern onto it.
http://www.napaonline.com/Search/Det...499_0058761886
Used a pair of tin snips and a hole saw to cut it out.
If you try the grinder and paddle wheel route, start with a high number grit as these can take metal off quickly if you're not careful.