While we don't yet have a dedicated case for the Arturia Rackbrute 6U, one of the cases in our UNISON line may be a good fit!
The dimensions we have on hand for the Arturia Rackbrute 6U are: (19 x 13.2 x 3.3) inches. If that aligns with what you have, we would recommend looking at the Custom Edition XL: (24.5 x 13.5 x 4.25) inches.
