A couple of flying seasons ago, I built a bunch of air powered rockets, including a many from AlaskaPaperModelWorks. I also converted one of their large paper Honest Johns to fly on 24mm motors. Late last year, I found that the pressure tube on my Saturn V was warped badly. You can see the warp in this photo (the engine bells themselves are warped, but that happened on landing).
I have decided to convert this for flight on BP motors. Probably 24mm but I may adapt down to 18mm. It has a full length pressure tube so I had to drill/cut/grind a hole in the top bulkhead and install a suitable motor mount. I decided on a full length stuffer since the cardstock pressure tube probably won't like hot ejection gas.
I found a tube that fits well in the pressure tube. That tube will have a 24mm mount attached on one end and a 24mm on top to fit easily through the rather rough hole I made in the top bulkhead.
I found a CTI reload tube fits nicely in the top of the body section. A piece of BT-60 or 38mm tubing will be mounted in the nose section. This will serve as a shoulder and will slide in the CTI tube. The components in the 2nd photo are roughly laid out as they will be mounted. Except the top 24mm tube is backwards. Oops.
Glue is drying on my quick and dirty centering rings. More later.