Position a seedling off centre in a pot or planter bag so that the skewed space created is big enough to put 1 or 2 smaller pots on top.
Stack up tubes on top of a pot or planter bag around a plant. Stick bamboo skewers in the tubes and tie a string around the skewers to keep the tubes upright around the plant.
Partition up a pot into halves or quarters using cut-offs from plastic bottles. For each pot size, save one cut-off as a template for future use. Seedlings growing inside the partitioned soil within the same pot are easily separated for transplanting or repotting in future.
Use heavier soil at base of a pot or a planter bag to retain water.