So in this GB, I'd propose Acrylic and PolyCarbon, an Dura-Aluminium. Other 4th, 5th.. are just private clumsy experimental stuffs.
For Acrylic Cases I'd like follow the Apple G3, G4, G5 Cases introduced in 1998, 2002, 2004... Hello Cuties!
Not a fan of Apple now but in my younger days, I would of killed to own an Apple inspired keyboard
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Dura-Aluminium looks good, got to see some pics on that.
Any extra 4th or 5th experimental cases made out of glass or even granite (yes stone) could be used. I would pay you for that stone keyboard case
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Here is another idea I've yet seen anyone do in the WORLD today. Make a mold (2-part) easy to assemble yourself and mix some cement/adhesive mixture and pour into the mold. Once it sets you should have a pretty durable cement style keyboard case. Only trick is to really make a durable case with internal supports to prevent any cracking. I know people in Europe pour secret mixtures into molds to build really delicate figurines and such, which are extremely tough (not easy to break).