I decided to figure out a way to adapt one of my earliest post-BAR 24mm birds, the BNC-R1, for the 29/60. I first Dremeled-off the spring retainer. I found that a BT-70/24mm plywood centering ring was about the same diameter as the body tube. Thus, it would work well as both a thrust plate and a mounting point for an Estes PVC retainer. I mounted that retainer to a short piece of BT-50 and the ring, just as if it were the end of any other rocket. I then sectioned another piece of BT-50 and slid it over that tube. This sectioned piece was measured to it would overlap the existing motor mount. I also found that the packing tube from 38mm CTI reloads was close to the ID of the BNC-R1's body. I added a short section to provide more 'bite' between the new bulkhead/centering ring and the old one. All these parts were epoxied together and, tadah, the BNC-R1 will accept the 29/60 and anything shorter.
Luckily, the old rocket was stable enough to accommodate the extra parts, larger motor, etc.