I'm looking for 2 or maybe 3 plastic barrels to fill with water to act as thermal mass in the new greenhouse I'm building (pics to follow in another thread), preferably in black. The basic idea is to store the heat generated during the day and allow the barrels to radiate heat at night in the winter to reduce the amount of heating required. We currently have 2x55 gallon steel barrels that are pretty much at the end of their service life (rusting through), and want to replace them with similar, only more rust resistant. These do not need to be food grade, but do need to be clean and non-hazardous. Does anyone have a good source?