Theres been another thread somewhere where someone stated that a guy took the test to an extreme with a 10 gallon, filling it completely with rocks then standing on it. I think you'll be fine either way. It'll give you some reassurance, but probably isn't needed. I think egg crate would be a good way to keep slopes intact over time by stacking it with the slope.
I would just do dirt capped with sand even with the egg crate. Roots will still go down to the bottom. I feel like having sand at the bottom will increase the chance of the substrate turning anaerobic, thats just my opinion so anyone correct me if Im wrong.
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