After my 5x Mosquito crashed in 2001, I decided to build a larger modular motor mount when I rebuilt it. The BT-80 tube had enough room for a BT-70 size motor mount. I got some BT-70 and couplers from Aerospace Specialty Products and built the mount pictured. The motor modules can accomodate 1, 2 or 3 18mm motors, 1 or 2 24 mm motors or 1 29mm motor. The 24mm single and double modules allow the use of 18mm motors with adapters. This gives me the versatility to fly the rocket on a huge array of BP and AP motors from A to G impulse.
I also built the mount into my small Crayon rocket so it can fly on the same array of motors. To adapt additional rockets, I only need a short piece of BT-70 and some rings which I fabricate as needed.
I have also built the same type of mount for smaller rockets using BT-55 tubes and couplers. Pictured below are my 2.4x Mosquito and downscale Fat Boy. It allows the use of a single 18mm motor or a cluster of 1, 2 or 3 13mm motors.