Hello Tom, there are more details on p193.
Wire is c.2.4m, space a metre apart, please read string explanation p193 (under the loops then diagonally across bed to hoop on other side), polythene about 2m wide and c.500 guage, old polytunnel polythene is good but I don’t know who sells 2m rolls of that thickness, do check out lbs.
Hope that helps.