I have a lot of very large pots and containers (20-50 gallon size) and I use
those Styrofoam packing peanuts instead of rubble, rocks, broken pottery,
etc. to fill the bottom. To keep the soil from sifting into the peanuts I
lay down a layer of coffee filters to cover them before covering with soil.
This is very helpful in 'lightening up the load' of big pots. Make sure the
peanuts aren't the biodegradable kind. If they get wet you'll just end up
with a small flattened, soggy, gooey, congealed mess. If in doubt run some
water over a peanut first to test it. Color and shape no longer distinguish
which are which.