I would use some cinder blocks to put a larger metal drum onto, that your plastic drum will fit into loosely. Put some bricks in first to keep the plastic drum from being in contact with the metal drum bottom. Put water in the metal drum so it surrounds the plastic drum (like a huge double boiler you use to cook custard and chocolate on the stove with). Put a heating device under it and let the water heat the plastic drum. No chance of melting your plastic drum and you can regulate the temp. Just don't let the water go dry.
