Use a little tumbler, build the pebbles around it, like you would build a well with stones. Cover the tumbler. Add tiny pebbles to the bottom of the tumbler, then fill with soil, plant!!
More pebbles beneath and around the tumbler would make it hill-ish. You could do several at various heights if you have a large enough aquarium. Tiered, sort of.
I can't think of anything else except clay and clay would have to be fired for permanency and to not mess with the soil.
