I haven't but ...
-Tape both sides of the shield with masking tape to cover an area more than 2"+ to each side of where you want to cut.
-Mark where you want to cut.
(These first two can be done with the shield still on the bike .... or not. The next steps, you might want to have the shield off of the bike.)
-Using a jigsaw (or something similar that will not heat up the area too much which will cause the plastic to melt), cut along the line you marked on the tape. Take your time.
-File and sand the edge to your desired smoothness.
-Remove the remaining tape.
-Install and do a final cleaning of the shield.
-Gear up and take it for a ride! If all is good, continue to the next step. If not, go back to the 1st step to make adjustments and do any additional cutting you want.
-Put away all tools and supplies used. Dispose of trash.
Well .... that's how I'd do it.