Hmm, I like the idea of the bottles ...
But I have used the styro blocks that are used to sandwich flat screen TV's or new ceiling lamps or plumbing fixtures into their packing boxes. These are not taken for recycling at mail stores so I just cut them up into pieces and try to fit them like pieces into a puzzle into the lower part of my big pots. I do this to minimize the amount of space that the wet soil can fall into. After fitting in the styro, I place a sheet of soft window screen or weed barrier fabric on top of the styro to keep soil from dropping down.
Whatever you do/use, the huge pots can be moved more easily if need be and less soil can be used if the plants don't have deep roots and if one needs the depth of the large pots to keep garden design to scale...
