If this is a propertiary standoff model it could be hard to buy. You can try designing and 3D printing similar part (with a hole in which you could place a metal nut), it shouldn't be expensive nowadays (I don't know how much commercial 3d printing services costs, but I managed to build a 3D printer myself for equivalent of 150 eur and the plastic filament for that single part would cost only something like 0.2 eur.).
But i think that you could try a cheap DIY-solution with bunch of M3 bolts, nuts and washers. You could use wide washers and nuts to mount a M3 bolt in the place of that rectangular hole, then just install the mainboard and put a nut on the standing out screw. I just grabbed some things I had lying near the desk and made photos of this solution. Imagine that this metal piece with holes i'm holding in hand is your case 😜 you could add or remove some washers to adjust the height.
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