I still have the old kettle in readiness here for the next time I have to steam a long piece of timber. It has a rim where the lid fits. I slip the layflat polythene tube over this rim, tie the string round to keep it in place, thread the tube over the timber, plug in the kettle with its old round plug and off it goes. Refill water as necessary through the spout. As it was winter we used old pyjamas as insulation around the tube. Pine vapour makes you woozy.
We did the boat timbers this way, horizontal so they would bend around the frames while still hot.. The last rubbing strakes we stood in the kettle outside the shed and it worked like a steaming flagpole..
Modern kettles, pah!