Can't find a plant that will work and need soil advice.

[Montium]

The other issue you might have is not enough light for the plant. Many of the popular plants for chams require a lot of light. There is also a product called feather stone that might work to root the plant in. It used to be used for bonsai fountains that were popular a few years ago. Depending on how much water you use, some of these plants can almost be grown bare root.