I run the same setup and while the bigger of the two intakes in the case kit measures out to 24mm OD and around 19mm ID you can certainly mill it out to 21mm and still have plenty to support your Dell 21
See here:
You can go to 21mm, maybe bigger. I stopped around 20.50 just cause I got tires of milling and my hands we hot.
The Dell 21 comes with a 24mm ID plastic bushing installed which fits so good on the Polini intake, so yeah, mill away Shane.