Time Bomb Movie Prop

I have recently been given the task of constructing a fake time bomb for an upcoming project. I drew some plans by hand, but since I don't have a working scanner I recreated them on the computer. They horribly suck but they should give you an idea of what it'll look like.
It has to have a working countdown timer so I went to king supers an bought an overpriced digital timer, not unlike this one:
I removed the plastic covering without damaging anything inside and also gutted an old alarm clock. I plan to swap the alarm clock screen, which is a red LED screen, with the current one. It'll look much better with it. I'm probably going to go to the hardware store and pick up some plumbing and wire for the rest of it tomorrow.