Thanks for the very comprehensive information !
I am using an Intel G3258 in the case with an ASRock Z87E-ITX board, so "Socket 115x". I'm guessing it's manufacturing tolerances for the heatsink, it might also be the PSU that's slightly out of spec. The heatsink has the pipes oriented at the right side, where the power button is located.
The heatsink sits right up to the edge where the PSU cutout is.
On the bottom right, you can see the difference in screw hole allignment. Now, this PSU also has no holes on the left side at the top and bottom corners, so it's useless anyways. But the top panel also can't close.
I'm looking at a Direct-Plug DC-ATX 160W with a AC-DC 160W, at the moment looking at the Dynamo Mini 160W (based on HDPlex DC-ATX 160W) and a 160W AC-DC (probably HDPlex 160W AC-DC). I'll contact you on the panels as soon as I get confirmation I can get these parts in time (before the 23rd).
For now this will be a CPU-only build and it might become an APU gaming PC at some time, which should remain well inside the 160W power budget.
