Also I wounder if anyone ever tried to build the #UltimateXbox360 and integrate the #HDDVD-ROM into the same case...

Like this one:
https://www.caseking.de/silverstone-sst-sg02b-f-usb-3.0-sugo-black-gesv-263.html

But I guess most of you would rather recommend me to install a front-mounted 2,5" HDD Caddy, but that would have the risk of users dismounting drives whilst in use...

So I guess a 2,5" toolfree caddy with lock to put in a PCI slot would be better...
- OFC it's only using the slot bracket, not an actual slot on a board!

That's not the #UltimateXbox360...
https://www.youtube.com/watch?v=FcX0pkXSKl4