You might be able to fill it with sphagnum that is often used as the "bottom" of hanging flower arrangements.
Or you could probably simply plant ivy at the bottom and train it to follow the frame you have.
I have the ivy plant now, and plan to use filler inside the deer.
Post a photo when it gets growing.
Douglas I will post pictures.
Hay. Stuff it full of hay. Hay bales are used in gardens, so should work well as a planting medium, in the deer as well.