While we don't yet have a dedicated case for the Novation Circuit Rhythm, one of the cases in our PULSE line may be a good fit!
The dimensions I have on hand for the Novation Circuit Rhythm are: (9.45 x 8.27 x 1.77) inches. If that aligns with what you have, I would recommend looking at the PULSE Case For The Elektron Octatrack: (14 x 9 x 2.5) inches.