While we don't yet have a dedicated case for the Casio SK-1, we have our PULSE Cases that may be a good fit.
The dimensions we have on hand for the Casio SK-1 are: (18.125 x 6.125 x 1.75) inches. If they align with what you have, we would recommend looking at the PULSE Case for the Arturia Beatstep Pro, dimensions: (18.5 x 7.5 x 2) inches.
The fit here might be a little snug so I could also recommend you the PULSE Case For The Arturia KeyStep.