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Here is a shot of the internal waist mechanic for the Cherudim Gundam, which should provide better poseability.
This is the 2nd version and there are still a few design issue I need to fix before I would print it out. This is all done in Blender and I find this process help getting the idea out of my head much quicker. However it's not that good at checking collision of parts. I might switch over to Fusion360 later.