Been working this one for a while. Based on a modification of Seattle Rocket Works MIRV Gryphon and Estes MIRV. Estes has the right idea, with 3 sustainers, each with a diamond shaped cross section of BOTH the rocket body AND the nose cone, all three "nesting" together at launch to create a single combined hexagonal sustainer structure. My issues with the Estes version of the MIRV were, first, IMO booster is underpowered. Second, I am not convinced the sustainers are really stable (although doesn't matter that much, as IF they deploy at a decent altitude of separation from sustainer, they won't go very far on A10-3T especially if they are indeed unstable) Third, sustainers are too heavy (which kind of goes back to first issue.) The "outside the box" (or in this case, perhaps putting it back INSIDE a BOX, just one without right angle sides) is the polygonal diamond body tube. Nose cones (technically pyramids) are also tricky, each is symmetrical right/left or yw, but NOT symmetrical up/down or pitch. So each nose itself is going to cause a certain amount of "off axis" lift. I tried styrene plastic for this. White Polystyrene 12" X 24" X .040" Plastic Sheet Styrene Pack of 4 ran me around $13 on Amazon. The stuff is not easy to cut (takes a lot of runs with Exacto blade, then you can flex in and break it cleanly.) Not impossible, at least for straight cuts. Angled cuts are challenging, curved cuts would be really hard (maybe a roll here for that $400 Exacto knife seen on another thread.) It's a bit heavy. My flight was unstable, although I think it got off the pad with decent speed, just not enough finnage, and again the nose cone probably isn't helping things. I am thinking of trying heavy Card Stock. Certainly will be much easier to cut, also can "score" the outside angles and therefore "fold" the body tube into the right dimensions. I am running a BT-20 down the middle, which serves as combination of motor mount as well as container and attachment for parachute. For the fins, I may just either double or triple up the cardstock, or reinforce with Balsa. The fin pattern I am looking for will be the same as the Mach Double Diamond https://www.rocketryforum.com/threads/double-mach-diamond.149662/ Anyway, differences from standard 3FNC include MultiStage MultiSustainer Polygonal Body Tubes Asymmetric Nose Cone/Pyramids.