I bought a cut off from technobots and it has a small rotary resistor with which you set the cut off point. It goes from 3.5 to 3V and even has a few little lights on it. I went without it last year but I may slip it in this time if there is space.

And David, we hole our Lipos down with industrial velcro. Its a pain to get them out but when they are in they are going nowhere! I had to Lever them off with a wooden ruler first time round. I was afraid I had got them stuck!