Reddit reviews JW Winco Steel 12L14 Tapped Nut, Knurled, Threaded Through Hole, M3 x 0.5 Thread Size x 7.5mm Thread Depth, 12mm Head Diameter (Pack of 1)
We found 2 Reddit comments about JW Winco Steel 12L14 Tapped Nut, Knurled, Threaded Through Hole, M3 x 0.5 Thread Size x 7.5mm Thread Depth, 12mm Head Diameter (Pack of 1). Here are the top ones, ranked by their Reddit score.
Knurled nut with tapped through holeBlack oxide finishMade of steel12mm Head diameterRoHS compliant
Thumscrews:
 
https://www.amazon.com/gp/product/B00A9NH22W/
 
I know it's cool to print your own, and these are a little smaller in diameter, but if you don't have M3 nuts readily available (and have Amazon Prime) and will have to buy nuts anyway, you may as well just get all-metal screws like this and be done with it. They work great on my MP-i3.
I've emailed my 3D printing friends to see if anyone will print a pair for me.
I've removed my Rift headphones and have the Knurled M3 nuts showing up Monday.
These match the ebay ones you shared