i used to bring mine to the gym and your harness should have at least a couple rings, on the sides and back. you can attach a low cable to the back, lying down on your back on a free bench and pull your head up. it/s difficult since you're lying down you have no momentum. try hitting different angles on the cables. I used to take a 45 lb plate and put it on my forehead while lying on my back, same principle as the other one i described, except this one leaves a real nice red ring on your face lol.
lay down on a bench or something so that your head is off the bench, hold a 10lb plate on your forehead, let your head all the way back, and then bring it all the way up until your chin is on your chest.
but i dont think your gonna find anything that beats rack pulls for getting a bull neck.
I train my neck with the neck harness once a week wit at least 50 reps and go no heavier then 25 plate, the reason why is i've tried heavier weight and less reps and pulled something in my neck. Needless to say it wasn't anything nice so 30-50 rep range along with deadlifts,squats, once a week you'll see some decent improvemnets..........