I will vouch for downloads magring2 kit. That thing is awesome. The fit and finish on his stuff is the best you will find.
A 3C maglite is going to pose the problem of having to use an extra long spring to make contact. I am about cheap builds, so if you have a 3c, run with it and make it work, but if you don't, then buy the 2c, you can get them at almost any home depot and I saw today that Bass Pro Shop also carries them.
Doing the simple math, a C-Cell battery is 50mm long, so in a 2C you have 100mm to work with. Obviously a 18650 is 65mm long so you are looking at 130mm, believe me when I say order downloads kit, 30mm or 3cm difference is well compensated in his product.