To echo BigC, yes 2 18650 cells will fit in a 2D M@G. You will need a sleeve to keep the cells centered. If you search this forum you'll find all the info you need.
I have made a spacer to keep the cells centered so that is not a problem. I am guessing I just have to relocate the spring to the bottom cap, and remove the anodizing.
Using a KD golden shorty spring I can fit 2 of KDs 32600 cells, that gives me a good couple of hours run time from a ROP low bulb. I can get over an hour from a ROP high bulb, but have to use unprotected cells so they will turn on the high bulb.