The best advice I have it to purchase a soft bag that is approximately 1-inch larger than the keyboard in all directions, width, height and length. Then, line the case with 1-inch thick Ethafoam, which is a closed-cell foam that is semi-rigid and very durable. The sheet that fits into the top of the case should be just 3/4-inch thick, which can then be covered with a 1/2-inch sheet of closed-cell foam rubber for added protection of the switches and keys. Then you'll have all the support and protection of a rigid case, but not the weight. I have a case made this way and have been using it for nearly 10 years. It looks and functions as well as it did the day I bought it.
Good Luck,
Gary
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Travlin' Easy
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)