I need thermal mass requiring minimal interior space in a low cost passive solar home.  I'm looking at multiple layers (3) of green board or cement board.  Can someone offer advice on the choice of these or other (thin) solutions?

Have you considered putting your thermal mass into the floor? This would provide a large thermal mass and take up little or no interior space.

A company makes clear fiberglass water columns that store heat and look pretty cool.

What are the cost numbers? That is just as important and energy reducing figures. At what price? Does it have a decent payback? There are only so many people that want zero/very low energy consumption at a price that does not make financial sense.