I have a resin printer and a placeholder project plan to design threaded retainers that are much slimmer than the Estes retainers. Target will be to make 24mm no larger than BT-55 and 29mm no larger than BT-60. Should be doable. No guarantee a filament printer will be able to make the parts reliably.

I've had a mishap with an Estes retainer when I put it in a lathe to lighten it up. Thinned down the cap in the Z direction a little too much. Super-secure at room temperature, but after one launch, I picked up the rocket and found no motor in it. Not an empty, but no motor at all. The cap was securely screwed on and the flange that should have held the motor in was intact, looking just like it should with all the exhaust residue and burning on it. Solid, no way for the motor to get past it. I can only imagine that the cap was softened by heat during flight, had enough flex to allow the motor to eject through it when the charge fired, and returned to its non-deformed shape before cooling during descent. Fortunately, the pressure required to do that was also enough to get the laundry out. So I had a perfectly nominal flight, except for the D12-7 somewhere out there on Lucerne.