In addition to the illustration mentioned earlier on page page 23 of Weaving and Cloth Design by Marianne Straub, you can see an illustration of a weighted box on page 105 in Ideas in Weaving by Anne Sutton/Diane Sheehan. There is also a short description of how to set up a weight box  page 60 of  The Technique of Weaving by John Tovey.
Isabelle