While we don't yet have a dedicated case for the Ketron SD-90, one of the cases in our UNISON line may be a good fit!
The dimensions we have on hand for the Ketron SD-90 are: (16.14 x 10.04 x 4.41) inches. If that aligns with what you have, we would recommend looking at the UNISON Custom Edition: Compact: (15.1 x 13.7 x 4.7) inches.