The 6XX come in a nice hinged box (although it is humongous!). My 598se were inside a box with cheap molded plastic to hold it in place (i.e., not really a storage option).
I decided to order some cases from Amazon to find a good one. I ended up ordering 9. Six claimed to fit the 600, or 598, or both, and three were smaller (I have some Sennheiser 202's and Philips that I would also like to be less cluttered in my headphone cabinet).
Only 3 fit the Sennheiser 598 or 6XX (they are different in design, but in the end seem to need the same space), and two of those were sloppy oversize.
Of the other three claiming to fit, one wasn't close and two could be made to fit if you flex the case and zip, but that is not very good protection id they are in compression.
This one was the best fit of the lot. The 598 or 6XX nests in it very nicely!
My reservations about it are:
1) The zippers are a bit tight as they pass over the lower corners below, so I wonder about long-term durability. It has dual zippers, so I will probably just zip to the corner on both sides and call it good.
2) It is not flat for stacking - not something I need, but always like containers that can be stacked.
https://www.amazon.com/gp/product/B01CVT9FQC/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1
I thought this was the better of the oversized ones (which might be useful if you wanted to carry some type of player or mini DAC, etc in the case).
If I kept this, I would want to add some padding to keep the phones from flopping/rubbing if used for travel.
https://www.amazon.com/gp/product/B01CHZ61FI/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
Surprisingly, I did not find a good fit for the Sennheiser HD202