It's a somewhat thinner slab of a huge tree trunk, Kate, with log sections arranged around and above a hidden water reservoir for the fountain. The slab of deadwood already had somewhat spongy fissures for root penetration, so all I did was to lay the sedum mat on top and see to it that it didn't completely dry out within the first few weeks.
Here are a couple of early pictures. A very simple plastic tote worked just fine for the above-ground reservoir.
I'm glad I tried it. The experience taught me quite a bit about using recirculated water for plant maintenance; well above and beyond the pleasing aesthetics of tinkling water.
I can't wait to get started on the new vertical project!
I'm thinking of a fairly deep, maybe 12" or so (this depth helps to keep the pump submerged), long and narrow reservoir base, perhaps constructed of wood, with the frame mounted to the back. A thick plastic liner might do for lining the reservoir, as I doubt I'll be able to find a tub in the dimensions needed to fit the space.