How about using old calor gas bottles? theyr'e a lot more durable than a 25L barrel. easy to cut up and weld too if you have a disc cutter and welder.
you need to wash them out thoroughly first though or you may get a small bang when cutting it open!! less risk than a petrol tank though as theyr'e generally less corroded so less like a sponge inside, and the gas is very volatile so doesn't hang around once the pillar valve is out for a couple of days and the cylinder inverted in the sun.
i got some from a place around swansea somewhere that reconditions them, if they fail hydraulic tests or are over a certain age they burn a hole in the side of them to make sure they can't be re used and weigh them in.
Dont use them if you are at all unsure about cutting one up safely, and dont use one thats still the property of calorgas