DMT Mini. These are small enough to fit two in there, probably. That would let you have a few different levels of grit. I suggest breaking the stone in first on a junk knife. These things have a real bite to them at first. Once broken in though, DMT stones are awesome. They are the longest lasting diamond stones I have come across.

I usually use a very fine ceramic stone to polish off the edge if I want to shave hairs with the knife.

But, it is debatable if this is the best way to keep an edge anyway. There are some out there (including me) who believe a slightly less polished edge acts as micro serrations and is more utilitarian. I have seen test of rope cutting where a knife that could shave took longer to go through a thick rope than the same model knife that couldn't shave. There are many many sites out there devoted to sharpening, even some pretty lengthy books.