I don't think it really matters what they are growing "in" Tiffany. Most Tillys don't have much in the way of root systems anyway. So for the purposes of display, you can put it on or in just about anything. The critical thing with them is to water and feed them through the foliage because they have so little if any roots, and the roots are mostly for anchoring the plant, (usually not very well) not for water and food absorption.
Mine is sitting out near the door to my pool cage, on a hunk of oak branch which is sitting on the ground. It's happy but I recently went too close with the leaf blower and it almost became detached from its perch. It's been there at least 3 years and still not all that firmly attached.
