One of the first upgrades for the Mini Mill was a set of 3 DROs. They are not expensive on ebay and work just fine for my use. I did have to replace one cable and I do have to remember to turn them off when not in use.
The standard mounting brackets were used on Z and Y axis but the X axis required a custom bit of aluminum to hold the transducer. Works like a champ.
I found the best place to mount them out of tool and chip harm by making an aluminum panel and arm. I used the hole in the mill head that was for part of the spring counter balance. I had removed the original counter balance and added an air spring. That gave me more height by about 2" and I need that almost every time I use the mill. So far the air spring is working flawless. It has been use about 2 years.