As @Ember suggested, check out using a cooler. For a specific cooler, search the forums here or talk to any homebrewer. The temperature ranges used for mashing (soaking malt in hot water to convert starches) are the same temperature ranges for most sous-vide cooking. For long cooks like lamb shanks, I prefer a cooler over a metal pot or plastic box for the extra insulation.
If you are still worried about leaching, go with a pot and wrap it in Reflectix (foil coated bubble wrap) see Insulating a pot using Reflectix