Adding a gap to a core for an inductor
Just insert non-magnetic shim material between the cores. Consequence is that you'll get a bit more leakage of EMI out the sides (flux fringing). You will be adding two gaps so you should use shim material thinner than with a gap at the core post. This is commonly done when prototyping flyback transformers, in fact you can buy special 'spacer' glue to glue cores together. In your case, I suggest procuring a kit of various thicknesses of plastic shim sheets used for mechanical engineering purposes and use the usual spring clip to hold the cores.  

It's easy enough to calculate the required air gap for the desired Al. See, for example, Transformer and Inductor Design Handbook, Col. McLyman 1-18~1-23. 

Grinding ferrite is not particularly easy to get right, even on a milling  machine, at least that's my experience.