Digicharge 19X23CM headphone case - ebay item no.203497721437, Amazon B08Y5XCJ79 . You will need to unplug the cable. The strap with Velcro keeps the headphones firmly in place.

I just tried my DT990 PRO in one of these, a perfect fit except for the non-removable cable. Your DT900 Pro X has, I believe, removable cable and has the same dimensions as the DT990.