I did find some specs and left 34mm for it. Theres still some room so its possible to make it more but not 6 or 7 cm which is needed to angle the standard straight plugged cord.
Its not a problem about the space for the angled plug but the space for the connector on the back side. And how to arrange the cord inside the case.
Again it looks like steambox prototype had more space thanks to custom stuff packed inside.
Is the space above the video card being used for something? If not you could shift it up and do the power cord like the Valve design.
Also you stated earlier in this thread you thought the original steambox prototype had a custom rear cutout, its actually standard. The positioning is achieved with 2x risers. a 90 degree one, and a straight extender to move the card 1 slot length away from the motherboard.
Maybe it changed in later models but you can see in iFixit's teardown that it's a custom bracket on the card: https://www.ifixit.com/Teardown/Steam+Machine+Teardown/20473