Still working on my new Shapeoko 2 build, but I wanted a set of pen holders ready to go when I do the initial tests. Wanted to use the stock spindle holder so there's no need to monkey around changing out the mounts.
Simply print twice.
Mounts with M5 hardware and uses M3 nut and bolt to hold the pen.
The mounting holes are off by a touch, but they are also oversized. Start both screws before snugging it down the whole way.