Drum? Acquire new 55 gal drum from local business. Make several holes and / or tears in the sides. Fill with water, have fun with plug and patch kit. Also consider getting some hazmat labels from businesses to apply to drum for identification drill.
Cylinder? Acquire a gas cylinder from your local gas supplier. They usually have damaged or out of spec cylinders. Make sure it didn't contain any Tetraethyldeath like poison inhalation hazard materials. Inert cylinders like Nitrogen work great. The gas supplier may even supply you with additional valves. Ask the supplier to de-valve it for you. Their de-valver will save the backs of 10 of the biggest truckies. Cut a door in the side of the cylinder. Hook up an old SCBA cylinder inside to simulate vapor leaks through the valve, neck ring, pressure relief device, or sidewall. If you wish to get really creative, rig for a liquid leak also using water.
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