I always hated the tube-slit method of shock cord anchoring so i used a version of the "shock lock" method G. Harry described in early editions of the Handbook.
Instead of the bulky trifold method which could hang up the laundry going out (especially on BT-20
or smaller) you simply cut a piece of scrap body tube about 3/4" square, cut two slits in that piece, thread your shock cord through that, then glue the whole kaboodle to the interior of the tube.
The curvature of the scrap of body tube will allow it to conform closely to the interior of the rocket airframe. Use enough glue to cover both slits then press it down firmly with your fingers and allow to dry.