I agree about finding them near man made debris. I found an eastern last year inside of a bag of soil in an old greenhouse . In this area you can see garter snakes and mice running around daily. I noticed that it was very warm weather and instead of basking the snake curled up inside a very nice hot bag which I would imagine keeps its temperature over night.
I think they may be hard to find at night with a light since they could be ambushing or searching burrows. To me the best way to find them would be to setup some boards or randon debris to give them some daytime hiding spots as well as habitats for their prey.
I also think the weather will play a big role in how active they are, especially at night. To me they would only be roaming on the warmest of nights unless they are very young. Also time of year such as breeding season could be a big factor as well. Around this time of year you may find one roaming at night who knows.
Good luck and keep us posted on how you do. I would like to find more milksnakes but they really do seem hard to seek out.