I had considered tupperware but I would prefer a solid media to prevent anerobic bacteria or bouancy problems. I may just get some lava rock gravel (2"-3" grains) from the garden shop. It's lightwieght, solid, and sinks. Mixes with gravel, it could provide a solid base for terracing. It's nt nearly as light as a hollow container but it is much less dense than regular gravel.