Yep, just fill the extra space with Ethafoam, which is fairly inexpensive and used to line keyboard cases years ago to make them more rigid, while at the same time, providing protection for the keyboard.
Gary
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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.)