The phenolic liner from a Loki 75mm motor is the perfect size to accept 12oz aluminum soda cans...just a tad off from a 2.5" body tube. I didn't want to use the liner and briefly pondered building an 18mm rocket from BT-60. However, I ended up rolling a tube from poster board, 3M "77" spray adhesive, and using Loki casting tube as a mandrel. Quick and sticky. The spray adhesive seemed to work well...until I sprayed on some primer, which promptly caused the tube to start peeling. A little epoxy and a strip of tape took care of that.
I cut a 7.5" length for the induction tube. This fits the rule of having that tube over 2 calibers in length and allows two cans to fit. Two cans with the top and bottom lopped off, that is. I cut one end down on the taper so it would overlap the 2nd can slightly. These photos show the induction tube with the cans installed. The two small screws allow the cans to be replaceable, will ensure they stay in place and, hopefully, won't affect the airflow. On the other end are 4 popcicle sticks that will hold the upper and lower body pieces together across the air gap. The tie wraps were only used to dry fit the tubes.
This is a quick and dirty build so it required a quick and dirty nose cone. This one is made from a cap from some spray paint, a ball from a ball pit, a cardstock shoulder and some braided twine.
I didn't photograph the upper section. I made it 11" long and ended up using a 24mm mount because I found the required parts i.e. didn't have to make any rings. An E9-4 should be a good motor choice. I added a Kevlar leader and will use some good old Estes rubber band cord.
I should have some photos of the finished rocket later today, complete with its fugly paint job. I think I'll actually need some mass on the bottom to get the CG just above the air gap. Note to self...glue on the launch lug!