Here’s my simple clock setup:
I’m using 2 6V lantern batteries and have had no problems with the 3V difference over the recommended 9V, and the clock has been for around 24 hours straight.
I need to connect the wires to the battery terminals better, but I ran out of electrical tape; still works well with masking tape though.
To convert from a wall power supply to the batteries I went to Radio Shack and picked up a size M (I think) DC plug (2.1mm center positive) which is the same as the power supply. The it was a very simple soldering job to connect it to the wires. I have masking tape holding the board to the batteries and it’s holding well.
I plan on using it for paintball scenario games, so I may end up coming up with something more complicated, but this simple setup works well so I’m not sure if I’ll change it much.
ENJOY!