This is the best light you bought in your entire life, and the best you
will ever buy in your life.
There is an endless list of mods available.
I would save those extra batteries you have got for eg. single cell CR123A-based (LED?) lights.
Or, according to the "one is none, two is one" saying, buy an extra E1E :twothumbs
Then, you'll have the smallest AND the largest SF light

(you can add a Lumens Factory lamp later, so you can use rechargeables in the E1E too).
The very first thing you should buy is a "Mdocod battery holder for Surefire M6".
The next thing is to buy two (or better four) protected 18650s in case you don't have these already.
Make sure you ONLY use the MN20 lamp assembly, (or the MN15, which is great indeed, but NEVER use the MN21 with 2X 18650), then put two 18650 in this holder, load it, and.... you will actually start to USE the M6, not being afraid or the otherwise HUGE running costs :green:
With the excellent AW 18650-2600s you'll have a run time of around one hour.
More modding options? If you want more throw and more overall light output than the MN20 delivers, get yourself a "3X17670 holder" by either Mdocod or FiveMega, then add three AW 17670s and a Lumens Factory HO-M6R lamp and see what this combo can do! It is regarded by many here on the forum as their favorite setup.
Do not use MN15/MN20/MN21 with this 3-cell setup! These lamps will suffer an instaflash.
What else? As soon as they become available again, snag yourself at least two Fivemega MN-bi-pin adapters. Then buy a few WA lamps (2-cell: either WA1111 or WA1274) or for 3-cell: WA1166 or WA1185.
I'm sure, which way you will turn, it doesn't matter: you will enjoy your M6!!
Timmo.