What to fill a plant pot with?

You can stuff large styrofoam pieces or empty plastic jugs into your container before adding dirt.

styrofoam/packing peanuts still get drainage needed but much lighter weight than rocks(put the peanuts in mesh bag if you don't want them loose in pot) ; you could also use one smaller plastic pot upside down in bottom of tall pot & another pot on top that holds plant closer to top

Plastic bottles, packing peanuts.

If it is tall and narrow, add some rocks or stones to the bottom to keep it stable. If top heavy it could blow over even in the slightest breeze. People use packing peanuts, empty water bottles to fill the void before adding soil.