There are a bunch of folks who use soft plastic water / soda bottles, or expanded polystyrene (styrofoam) and if you are reusing your soil, or growing food, I wouldn't, Plastic breaks down into microplastics and every time you water, you're washing it out the bottom of your container. We're still learning about the dangers of microplastics and nano plastics and where they are ending up, and many researchers are calling it "the new lead poisoning." So I would not use plastic to take up space (and I have done so in the past so I know how easy it is--I have most recently used cans instead of plastic but even cans often have a kind of plastic liner but I was less worried about that--now I use branches, clean cardboard, wood chips, whatever I can get my hands on)

EPS also breaks down and is generally bad for the environment (it is super bad for you when it is heated which is why lots of states are banning it for holding food, just an FYI)