Hello,
I'm wondering if anyone can recommend a cutting bit (preferably with a 1/4" shank) that mills nomex honeycomb nicely.
Thanks!

I forgot to mention that density of the honeycomb matters a lot.  Heavier material will be more stable and easier to machine but under 40kg and it will be a mess unless you have the right tooling.  

The burrs will - in my limited experience - work but it will be nowhere near as nice as the special cutters in the links we both posted - Neuhaeuser and Corehog.  You have to go faster than you think feed-wise and it will tear up light honeycomb.  The burrs work ok because they are almost abrasive instead of trying to take chips like a fluted cutter.  This is way easier if the honeycomb is already bonded to a bottom skin... but doublestick tape, temporary glue and maybe this product:

https://www.airtechintl.com/blog/new-corehold-p-ha

or something like it will help.

But yes, Rohacell is very nice for this kind of thing!