These are some modifications that I have done to a WitEden 2x2x2, and I am not aware of other mods existing for this cube.
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The Shuang Ren core is the best design that I've seen, so I used it.
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This inner edge mod keeps the cube from locking up from internal pops on looser tensions.
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I split the tabbed edges in half, and glued them to the centerpieces. I then filleted them where it was needed.
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This is my core assembly, with the moveable centers having 2 washers with rounded sides facing each other, and the bandaged centers having no washers. The screws are from C4U. I pressed nylon washers on the center stalks so that the pieces don't sink into the puzzle. They also reduce internal pops.