On the above loom, my Leicester Dryad countermarche type, the back beam and warp beam do dual duty. The warp comes off the beam from the outside, so the heavy weights are attached to the outside end of the tension cord. This loom has 10# of weight on each side, counterbalanced with 2# and 4#. (I probably ran out of 2# weights.)
I do not recommend using water bottles, milk jugs, or other thin plastic, since many are designed to break down after a time. I didn't know that until a couple milk jugs that I was using deveolped slow leaks. I stick with solid items or sand for weight now.