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Decided to try and tackle Brawl's tight-ass transformation to make it a little easier, and the major thing I noticed about the arms being too tight to tab in is that they tabs just straight up aren't lined up with the slots.
I'm not sure how well it shows on the picture, but you can kind of see how the tab is almost a millimeter off from where it should be, and how much I shaved off from the tab to make it fit more easily.