There are any number of ways to attach the shock cord. Your plan to attach the shock cord to the top centering ring is one that many folks use. Some tie the shock around the motor mount and run it up through a slot or notch in the top centering ring. Others use an appropriately sized eyebolt drilled through the top centering ring and bolted in place from the back.
My own preference is to drill a hole through the body tube just below the nosecone and run the shock cord out through that. I then knot the shockcord and cover it with a fairing made from a split plastic nosecone.
All of these methods work.
Kit (AKA Cranky Kong)
Total Total Impulse as BAR: 7,753.69 Ns (Equivalent to a 51% M motor.)
=| Calirado, Colofornia...what's the diff anymore? |=