I had trouble getting the cables to connect to the terminal blocks on the ends of the sonoff board because I have an older house with old metal boxes. So, I made a modification to the way the power lines are connected to the sonoff.
1) I removed the terminal blocks and soldered stranded extension cord lines to the traces on the bottom of the board.
2) To accommodate the cords, I simply drilled holes in the back of the case for them to slip through.
NOTE: I had numerous issues with my two test switches restarting randomly. This was ultimately solved by changing the version of the board driver I was using in the arduino IDE. I was using the ESP8266 2.4.1 board driver and downgraded to 2.3. It made an incredible boost in performance and reliability.