When I stopped at Carter and Holmes a while back I fell in love with a young plant still in a flat and I convinced the owner to let me have one. Since I was traveling and it was bare root, I also bought a mesh container and just enough medium to pot it up. I had asked them about the pot size and the best medium for it and they gave me the shredded cedar. I like the material, it is lightweight, pretty finely shredded and cedar would naturally repel insects. I don't know if it will last as long as fir before it breaks down but I should think it would be fine for your purpose Ken.
Fiat, I was told by a grower that the key to keeping the Zygo happy was enough moisture, they prefer a bit more water than most. I potted mine in the Better Gro stuff you get from any Lowe's or HD and it has been a very happy camper.
