Re: 1000+Lumen Malkoff Quad in Duracoat Host! **2nd REDUCTION**
I make my battery sleeves with a variety of plumbing and irrigation parts from Home Despot, and o-rings when needed. It completely insulates the cells from the inner wall of the flashlight. It would fit perfectly, and hold the cells perfectly centered, but it wouldn't be as pretty as a lathe-machined delrin tube like the kind wquiles makes.
As I said, you'd also need a short spring mod, because two 18650s are longer than 2 D cells. This means you'd have to change springs if you were changing from C or D cells to 18650s.