If you can fit it in your messenger bag, I would just keep using that. It's versatile for on and off the bike and it keeps your gear close when you're off the bike grabbing lunch or taking a break or whatever.

If you need a rack, I'd suggest a rear rack with a pannier bag. Preferably something removable that you can take off whenever you're not using it. That's sort of what I'm looking into now for my Masi Uno.