But, I don't know what I'm doing, honestly. I don't know if the sphagnum moss will degrade over time and if it does I don't know what to do about it. I was told I needed it by a gardener friend but topiary isn't his specialty. I just want this to last.

Does anyone have any advice for me on how to make it work and what sort of maintenance I'll need?

Oh, I took another look at the photos and I see the fig is planted in a pot at the foot of the topiary frame not actually in the moss. Will the topiary be kept indoors? I've never grown creeping fig but if it's like an ivy I should think it will cover the frame eventually and then the moss won't be needed anyway if it's rooted in the pot below.

Now, what is the purpose of the pot in her chest? Is it to be planted with another plant later? If so, that's a unique idea.