A sharp exacto knife will do the job.
To start with, you want to cut the stack just above the bottom rib leaving a few mm of rubber above it. Then shave material off above the rib gradually as you're working your way around - until you get level with the top of the rib (this will be the top surface of the cut-down stack and allows you to put a nice internal curve on it). Then do the same on the inside until you get a nice curve. The trick is to take very small amounts of rubber off with each shave. You tend to loose a little bit of rubber where the airjet sits as it is very thin there when cut down.
Then sand with 400 grit (ish) wet and dry (dry) and finish with 1200 grit wet for a nice smooth and shiny finish. In the end, it doesn't have to be perfect, but smooth is good.
The pic below should give you a better idea - you can see how the rib allows you to make a nice internal curve.
Could have sent you a set a couple of weeks ago, but looks like I've binned 3 of them. Go figure
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