Yes, one option would be to cut around it, but I feel like that might make the hole too big for my liking, and looking at his pictures, his hole seems to be smaller (i.e. inside where the ridge should be, but there is no sign of a ridge).
As far as the hole goes, I was thinking ideally 10-11cm to match the size of the 120mm fan which will be inside, and to roughly match the real mac pro opening size. But only if I can find a way to neatly remove this ridge. It could take forever to sand/file, and I don't know how to go about cutting it...
edit: also, nice looking grille. that is the same sort of thing I am thinking for the top as it will exactly cover the fan, and will look neat from the top. I know I got a similar thing from the Laser Hive for my G5 mod, so perhaps I can do the same again for this...