Point of information: do you have to actually WRITE DOWN what the piece is when transposed down an augmented fourth, or do you just have to PLAY IT as if it were? Because if you just have to play it as if it were, you can just play it in the enharmonically equivalent B minor and pretend that that is C-flat minor. If you actually have to write it down, I don't know what customary practice would be but I've NEVER seen music written with double-flats in the key signature. And for C-flat minor you would need E-double-flat, A double-flat, and B double-flat in the scale.