For larger objects a ranging rod is usually good, but they're very expensive. I bought 2m of 16mm white pvc pipe and a roll of red insulating tape from B&Q, and got three ranging rods (1m and 2 X 0.5m) out of it for about ?3.50. Smaller ones would be even easier. They weigh nothing, are waterproof and very strong. A 20cm one would take about five mins to make and mere pence in cost. That said, the field scale card yrammy suggested would probably be even better.