Having perhaps once met someone in the distant past who may have used jerry cans to tanker fuel I can mention a couple of minor points. Jerry cans are prone to rusting inside and need to be carefully checked at the bottom and corners before use. After engaging the locking lug shake and turn upside down to ensure the rubber seal is working OK, the smell and leakage of petrol in flight, apart from making you feel sick, can constitute a health hazard. To get any decent extra range you need about five of them, 100 litres and it is hard in most light aircraft to find somewhere to fasten them down securely, even lined up side by side. And last, no matter how carefully you handle them they are awkward to handle and tend to damage the paint and put dings in your luggage space. Not trying to put you off or anything.