Model Rocket News Idea Box - Shock Cord Attachment
From the Estes Model Rocket News, Vol. 3, #1, Feb/March, 1963.
Here's an early shock cord retention using an engine block. It's similar to using just a segment of body tube used in the Enerjet Mercury Little Joe I.
This tip came from 1963 when the the illustration showed a minimum diameter tube, a BT-20 or BT-30. I wouldn't try this in small tube models. Tube segment retention works best in BT-55 tubes and larger.