As I hear all kinds of horror stories about Li-Ions all the time, I want to check this one out with you guys.
My TF flame 18650's are a bit on the thin side. I don't have the excact measures here now, but I have measured them to be a fraction of mm thinner than for example my Cytacs. Because of that the TFs rattle in most of my 18650 lights. I there any problem if I wrap a round or two of plastic electrics tape around them to increase the diameter to stop them from rattling?
The only problem is that electric tape usually retracts and leaves some sticky glue which collects dirt and the battery starts to look kind of ugly. I use a thin strip of ordinary paper and some scotch tape instead. By doing so I leave the battery look nice, and if needed I can slide the paper off and put it on another battery.
Because we all know flashlights is all about the looks. ;)
It is more practical to prepare the flashlight itself, rather than preparing the batteries. Just insert some layers of normal xerox paper and the rattling will be gone. This has the advantage that you can swap around batteries with all your flashlights, which might have different "gaps".
While on the subject: using o-rings if the gap is large works perfect. The Romisen RC-N3 for example uses either two AA or one CR123 battery so when you're using two AA:s the battery closest to the LED rattles like crazy. This is easily solvable by putting two o-rings around the battery. One downside in this example is that you have to put the battery in the "wrong" way since the the tube closest to the switch is to narrow.
Using tape also create a heat insulation from body tube to battery which might be very helpfull to the battery if the flashlight is hot. In my scorching hot XM-L the battery was always mildly warm as opposed to the hot body. I was always thinking my battery was cooking in there which is very not reccomended and unsafe for li-ions.