Clean terracotta and stones would be suitable for both the stability and drainage.Iif stability is not an issue you may use broken polystyrene, the sort used in packing. Don't use concrete, wet or set.!!!
you can use concrete. You can even buy tubs made from concrete. It is possible it can raise the ph of the soil but in small amounts it is unlikely to cause a problem. As long as you use broken pieces to allow for drainage and make sure there is enough room for rooting in the tub then you should be fine.
Yep...as with halkwalk....perfectly ok although i'd try to use older concrete rather than newly mixed. This will allow a lot of the lime to leach out and prevent the pH getting too high.
I've even made my own concrete rocks for my marine aquarium and as long as they're "cured" properly, they prove no problems at all