i have a stock screen i send you if you cover shipping and packing materials. its not perfect...some blemishes/scratches. i guess it depends on how flawless you want it to be.
as for your question, i would use a dremel with a cut off wheel. put some masking tape over your cutting area and draw the best cut line you can on that tape. after the cut, use some fine sandpaper to clean up any rough spots.