Hi Photograaf16 - I'll take you up on your offer for a smaller version. Mine printed out about 1mm too large on the Z axis of the four posts but should be no more than 7mm and I'd err on the side of caution and go 6.8mm. In fact, would you mind much if we mod the design just a touch? Instead of using a captive nut I'd prefer to tap the screw into the plastic and do away with the nut. It avoids what could be a weak spot where the nut sits, especially with the narrower bit around the nut. Tapping 12mm should more than compensate for the absence of the nut.
The rest of the design is brilliant and printed out nicely. I need to break-in the tension bolt a bit but that's normal for a printed nut and bolt and I had the same with the printed wingnuts I'm using on my stock spool holder.
Now I have to put my Y idler back together so I can print the new part. It's part of the fun.