I've also designed a digicam intervalometer. (see my sig.)
I chose to power it from an internal 9 volt battery, providing 5 volts through a low dropout regulator. (There are also external power provisions, with a regulator to provide camera power when running from an external 12 volt supply.)
Hantronix makes some nice miniature 16 x2 LCD modules. Sockets are available through Digikey.
Rather than try to fit an Arduino board into my (hypothetical) box, I've used a bare Arduino chip on my project board.
Although I haven't packaged my unit yet, the little Hantronix display and not using an Arduino board will let me build the unit into a box small enough to put into my gadget bag. I've located and obtained a suitable box, and might actually package this thing one winter weekend.
Good luck!
Tom