I use that sheath for my U2 and it works great. The open bottom allows for the long length and the "width" is adjustable. The head of the light remains above the sleeve and the adjustable flap covers it nicely. Even though the bezel is a bit larger on the M3, it should still work nicely. Maxpedition products really rock!
Also, to protect the bezel, and the stuff around it, check out a rifle scope cover. I got one at my local sporting goods store and it works great. Clear lens (can be frosted with steel wool) that flips open....many different sizes and cost un $7.