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The coupler tube would be bolt to a structural ring on the inside of the body tube that would hold it in place. This is the preferred plan.
Part of the coupler would go in between the body tube and where the avionics bulkhead is being bolted in order to constrain both of them with fewer bolts. However, the amount of load put on those bolts from the torquing of the upper sections of the rocket is thought to be less than ideal.
EDIT:
The decision was made to omit the avionics bulkhead entirely from the design as well and the coupler section of the rocket that served as the Avionics Bay. Now, the piston and the avionics tower will attach only at the payload bulkhead and hang down. With no coupler section, the body tube will connect to the nosecone via a coupler attached directly to the payload bulkhead that bolts into the nose cone and the coupler section. This design change allows the piston to not be in tension and only be attached to one bulkhead; has the avionics tower still constrained to the payload bulkhead; and still allows for a way for the body tube to be connected to the nosecone.