pop cans , 2 litre soda bottles with lids on. bricks, large rocks. i would not use card board it will fall apart. i would use landscape fabric or fibreglass screen material between the soil and filler.

I fill my deep beds with composting material: leaves, kitchen scraps saved over the winter etc. By late fall/early spring I have mostly composted soil for pots and beds. The mix of materials determines how many years until it is done. The beds do settle a few inches. Settling is a minor problem around the edges.

M - I would fill to desired ht with ballast, like empty soda bottles or milk jugs with screw-on caps and back-fill with no covering (like cardboard or landscape cloth) at all. I wouldn't fill the bottom with anything organic, as it can turn to a soup-like consistency if it's not very stable, increasing water-retention, reducing drainage, and producing noxious gasses you don't want in the soil. There are also some considerable benefits related to drainage and the ht of the PWT to be had by allowing the soil to filter down in between the items you're using for ballast.

Al

My raised beds are well aerated. I built them using old fence boards stacked like a rail fence. So basically it's board-gap-board. As Al mentioned drainage could be a problem. I'd drill extra holes in the side for air/drainage.

What about using cinder blocks as a space filler in stock tanks? Are there harmful chemicals in cinder blocks?

What about using cinder blocks as a space filler in stock tanks? Are there harmful chemicals in cinder blocks?

Rocks

I think cinder blocks usually have a lot of lime and could make the soil more alkaline.