Lacy, there are some really good, heavy, food grade white jugs that restaurants get cooking oil in that they just throw out in the trash afterwards. I've scored many of those jugs...most are around 3-4 gal., square, with a screw on lid. I had collected them at one point to use for just the purpose you propose in a small green house I was going to build. Folks paint one side black, fill them with water and stack them up so that the black side is absorbing heat all day and then releasing it through the white side into the interior of the room during the night. They last simply forever and can be used for so many things but their square design makes them particularly good for this purpose of passive solar heating.
Here's a pic of some.....

Here's a pic of some.....