Springy pen holder for MPCNC

[Allted's springy pen holder](https://www.thingiverse.com/thing:1612207) for the Mostly Printed CNC is a model of simplicity. But I didn't like the idea of having to cut zip ties to switch pens on my MPCNC. So I replaced the pen side with straps using M3 screws and nuts. The nut traps are likely to mess up a bit while printing, but just tighten up the screws to pull the nuts into position.

You can customize the maximum pen diameter (default: 18mm), and with long M3 screws, you can go as thin