Assuming you have not done much clustering with composite motors, I would recommend starting with a model rocket converted to a cluster mount. I’d try a Big Bertha, and put two 18mm mounts in it. Do some BP clusters first, then substitute the Q-Jets. Composites are more of a challenge.

Estes used to sell a Bertha-like rocket called the Ranger. It looked like the Bertha, but had 3x18mm mounts. One other differences was that the 18 inches of body tube included a payload section, where the Bertha is just one tube. But the Bertha would be something you can buy today, and could easily convert to a cluster.

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Lee Reep
NAR 55948

Projects: Semroc Saturn 1B, Ken Foss Designs Mini Satellite Interceptor
In the Paint Shop: Nothing! Too cold!
Launch-Ready: Farside-X, Maxi Honest John, Super Scamp