Well how I did it may not be the best solution, but I've used this method several times before with very good results although it is quite time consuming.
I take a pair of tin snips and cut out the grill leaving a complete, or as near complete ring of holes remaining around the perimeter as I can.
I then take and carefully snip out the webs between the holes leaving a single web remaining as the sole attachment to the case itself on that one small piece.
I have a really small pair of pliers with the jaws set on an angle to the handles. I carefully position the jaws of the pliers at the point where I wish to break off that piece. It takes only a few back and forth rocking movements of the pliers to break off that piece. It leaves a surprisingly clean break.
After removing every piece from the outer perimeter, I take a half-round file and dress down any sharp edges remaining. I carefully tape off inside using heavy paper to catch any filings, and use a shop vac before removing the paper and then the vac one last time.
There is still some distortion around the opening so it is not the best way to do the job. But I've used this method several times with the entire system still assembled.
I've got an air compressor, die grinder and carbide burrs which really do a clean job, but I am too lazy to disassemble a system once it's already in the case. On those really restrictive looking grates, where I know it will have to come out. I use the die grinder first off.
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