A small flat locking foot switch will fit fine above the assignable switches. Just use regular velcro not industrial. I used to do this on my other keyboards for vocal harmony on off. works great.

MEL
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KORG PA1000, KORG PA900, 2 BOSE S1 PROS, 2 BOSE L1 COMPACTS, YAMAHA STAGEPAS 500, ROLAND VP7