Convert that .3dz to a .3ds. Then dump the .3ds file with braindump, then use ctrtool to decrypt the .cxi.
I'm assuming you have a sky3ds since you have a rom for it. If you don't have a sky3ds, as far as I know, you can't do it. Or maybe I'm just forgetting something.
Just look around the temp a little farther. You might find your answer.
Sent from my Browser on a New 3DS XL with A9LH