How can I fill up a large and deep planter?

If you find rocks they will work.

Styrofoam packing peanuts work great. Water still goes through plus it keeps the weight down. Great way to reuse them too!

Place rocks or something heavy in the bottom. You don't want the finished container to be top heavy or blow over in the wind. Then fill with empty water or soda bottles, packing peanuts, sand, rocks, whatever you have, then potting soil and your plants.

Place some larger empty plastic plant containers upside down, in the bottom of your larger one, to take up space while reducing weight. Make sure it has a drain hole, to let water escape. To slow down water loss, wet some #4 coffee filters and place them around and over the bottom drain hole, having as little overlap as possible.

In a pinch I have put empty pop cans in the bottom...no weight but does take up room so you don't have to use so much dirt. As Olivia said, which I have done, used the plastic plant containers also.