Simple, Portable, Countdown Device Design

Forgive me for asking again for help on a worn out subject such as countdown timers. Normally, I'd take on the challenge of design using the many pieces of information already posted, but time constraints force me to find a short cut.

I'm willing to pay for the design of a simple, portable, countdown timer, capable of milliseconds, that sounds a horn/beep when countdown timer reaches zero. I have placed a crude sketch and requirements list below. I suspect the design would be simple for some of the vets on this board, given other devices that have already been built. I'd be interested in your feedback/comments.

Cheers!

Ben

1.0 Countdown Timer Specs.pdf (197 KB)

Function: Simple. Input the desired time to be countdown from.

Input it how? Using some application on a PC that can talk to the serial port that the Arduino is listening to? Put it in a file on an SD card, and insert that into the Arduino's SD card reader?

99 second countdown in milliseconds doesn't really seem reasonable.

How big a device are you thinking of? How long should the batteries last? Should the device be able to charge the batteries, too?

How many of these devices do you need? Fancy cases?