Hi everyone, I too just bought the Korg Microkey 37 and i found the perfect hard case for it. Try looking for tackle case/bag that is at least 60cm length.
1) 60cm lure Hard shell bag length Portable Fishing Rod bag Double Zipper Shockproof Storage. https//m.aliexpress.com/item/1005001409119608.html?
2) Outdoor Fishing Bags 63cm EVA Shockproof Fishing Tackle Bag Portable Rod and Reel Carry Bag Pole Storage Case with Strap PJ221. https://m.aliexpress.com/item/4000631402579.html?
I bought the 63cm storage case as it had the option of a strap & additional space for the foot pedal.