Guys, sanding or filing the Malkoff a little will be fine, even in an MD2. If you have problems when using it in another module, just wrap it in aluminum or copper (tape or strips) for a snug fit. This will help make contact w/ the body and ensure good heat sinking as well.
Weatherproofing is fine. That gap is only an aesthetic thing. Some members file their Malkoff drop-ins to reduce the gap. A complete bore through (no battery stop) is the only way to eliminate the gap completely. Of course you would have to wrap your drop-in w/ aluminum or copper tape for body...
If you can remove the head from your SF Fury, Fivemega has a body that's already bored for 18650's. That way you don't have to mess w/ the stock body. Also, the Fury works great w/ an 18650.
The Oveready extension does fit, but they seem to be out of the black finish. Nitemods carries a 68mm body (black) that will work too. The Malkoff MD2, MD3 and MD4 bodies have Surefire compatible tailcap threads.
I'm constantly changing drop-ins for my MD2, depending on what i'll use it for. I only use long nose pliers and never over tighten the hi/lo ring (just snug). I also don't over tighten the head when changing modes, and never had any problems w/ the ring coming loose. I do keep my threads and...