There is no advantage to using lemonade rather than water. Despite popular bankside beliefs, the maggots don’t take in the liquid so don’t take in the gas. They take in air to stop themselves drowning. That is what makes them float.
andylaw7 is right about the best way to do it but I would say that if you use lake or river water at this time of the year the maggots may well drown as the cold water will stop them taking on air. I do mine at home before going out with very slightly warm water.
If you can get your hands on an old ‘Colditz’ box they are the best. They are round and have 2 lids, the first having 2 lips on it that even wet maggots cannot get over.