I use the Accessory House DN8PRO, both the regular and XL version (don't have a DT 900 Pro X though).

I think the DT 900 Pro X should fit in either, but the non-XL would likely be pretty tight; the XL would be a bit loose but has a strap to hold things in place.

I had used the non-XL with a DT 1770 Pro, and while it fit, the (wider than the DT 900 Pro X) earcup forks left imprints in the lining. I had used the XL with that same DT 1770 Pro with Dekoni pads, and it fit with room to spare.