This is a casing that fits onto an M4 Nyloc nut to make it fit snugly into a cavity that's too big for it! It can be resized for any nut.

I'm printing the metric/"international" version of the MPCNC, using 25mm pipe. And although it's all marvellous, there is some difficulty with the fittings that grip Nyloc nuts, which are meant to enable you to tighten them in place. Specifically, the hex-cavity for the nuts were originally designed to hold US/Imperial #6-32 (3/16") nuts, but when adapted to M3.5 or M4 nuts, there's too much room and they spin freely - but there's not much room to get pliers in to hold them.

So I've designed these little cases. You put the nut in - quite a tight fit, and a bit easy to split, so print a few; I use a small hammer to tap the nut in - and then it fits the cavity far more closely.

Printing is a bit tricky because the casing wall is so thin. I used a 0.25mm nozzle for the first time ever. The design as supplied is aimed at an M4 (4mm shaft, normal 7mm spanner-size) nut and makes it fit an 8mm hole - which means the case walls are only 0.5 mm wide, or less to allow fitting.

In the photo above, you can see them printed, one of them has nut embedded, and you can see them in place on the MPCNC leg clamps (I put a black one onto a red foot, so it's clear).

You can read discussion about this issue at the MPCNC Forum. This solution has worked really well for me: problem solved.