Build your own photocell. It’s fun to use our own tools. There are many places on the internet but here is one:
http://home.earthlink.net/~lenyr/photocell.htm

Use it in different places like, in the middle of the town, out in the fields, etc. and register the level at the same conditions.
I would try to make sure that e.g. the cell pointing at the zenith. Then you should do it a new moon when the moon is not up. Last, you should do it at the same time of the day. Midnight would be good.

Good luck with your project!