Cut-cut-cut.
Today, a lot of stuff is gonna arrive, including:
- metal tin snips ("Professional Tin Snips")
- a Dremel mini-routing attachment
- some kind of "parallel cutting" helper for the Dremel (for straight cutting the top of the 2nd S300)
- something called a "Falzzangen-Set" (fold pinzers?) - its used to manually fold metal sheets etc. to specific angles (45°, 90°, etc.)
- a different PSU extension cable with the 90° angle pointing "down" - because the one supplied in the S300 doesnt go well together with most PSUs, at least if you dont want the GPU to breath inside of the PSU fan; you always have to twist it in angles that feel .. unsafe, to say the least (like: worse than creative tube arrangements in the A4-H2O 
).
On that PSU topic: Thats an issue with BOTH the S300 and the S400. I suspect its also the case for all other related cases, eg. the S450, G200, etc.
Why not just go with a differently angled adapter? Eg. still angled by 90°, but "down" instead of the left or right. Sure, in the S400 V2 the cable is routed across the mainboard, but in the S300 it would certainly make much more sense, because the extension cable would be routed through the GPU side anyway.

cu, w0lf.