Three 18650s fit very well in a Mag4C. The length is nearly perfect, the stock spring works fine. But a spacer is needed for the smaller diameter. I used some of the gray 3/4" PVC electrical conduit. It's a little too large to fit without modification. I just cut a slit in the PVC lengthwise and then pinched a piece of sandpaper in the slit and ran it up and down to take off small amounts of material. After just a couple minutes of sanding, test fitting, then sanding a little more I had a perfect fit. By removing small amounts of material with the sandpaper, I was able to get a snug fit where the spacer won't fall out when changing batteries and no gaps where the edges of the slit come together. I also drilled a hole clear through one end so I would have a way to pull it out if I needed to.
