Although its for the Ponte Vecchio Lusso, here's my story of today
I had just received a second portafilter so I wanted to make the old one a naked. I looked here and saw that this was possible with hole-saws or multiple small holes, all followed with sanding or filing. Before trying this, I went by the technical workplace of my university. There was a very nice guy with some spare time and a very nice machine that cut the bottom off within 2-3 minutes. Below are the results.
You might want to check up if there's no big machines and nice guys around your area - perhaps a 10-20$ fee for the work if they require it, but it might just be that showing respect to the big machines is enough


