Maybe some protected 17650 or 16650 cells... but you lose capacity vs. a good 18650.
Good Old TrustFire? At least they seem to fit whatever I've tried them in. 
so you can’t get them in or they won’t come out?
Well, I’m ashamed to admit it, but I got them to fit…. I took off the stick-on label, a light coat of silicon grease on the inside of the tube- and with the help of a wood dowel, by golly- I got ’em in there!! I do not expect them to come out as easily… or gracefully… or reuse-ably!!
I’m going to purchase a Flex-Hone and see if I can’t hone out the cylinder a bit… though I’m still awaiting a response from Malkoff to see what he has to say. I maintain this is a known problem, and he should have made the tube a half mm larger- to accomodate protected cells, which is what should be in a 2-cell tube!!
To add insult to injury, I ordered 2 sets of these batteries from Mountain Electronics LLC… but when the well-packaged envelope arrived yesterday, the end of the bubble mailer was torn off- and only 1 of the 2 sets of batteries enclosed! Had to be USPS fault. Of course, since the package was not insured, they won’t accept a claim, even though they mangled the envelope.
To Mountain Electronics LLC VERY GOOD CREDIT, I spoke at length with them, and they were MOST helpful, and are sending me another set of batteries, and they suggested possibly using 17650 or 16650 cells… which I’m also going to investigate. Would just like to say that Mountain Electronics earned my future business today FOR SURE- and I would highly recommend them!!
Wes
Flashlight manufactures should be allowed no excuse.
You can enlarge the battery tube with sandpaper & a wood dowel. budgetlightforum.com/node/18473#comment-324577
Hey just wanted to comment that this thread taught me alot. I wondered what the differences were in sizing & how to use the protected vs non. Now its clear to me. Good info for a newbie. Thanks.
Also, I have seen those magnetic spacers & plastic sleeves. I am assuming that those are to help if someone has batteries that don’t fit, they can use a smaller cells of the same voltage to fit their light. Seems like its kinda a ghetto rig?
be very VERY careful with magnets
if the magnet touches the side of the inside of the flashlight tube the batteries could short out - very dangerous situation if the batteries are unprotected
that's nice to know.
If you want a quick fix buy the 3400mAh EagleTac 18650s (at: GG, illuGear, LJ, eBay).
at 18.4mm (HKJ measurement)... these may work.
cf. also to "Diameter of Battery" 2nd Chart down. (HKJ)
yes indeed. I read that a short while ago. One fellow said to try and super glue them in place. Very nice to know. 
I just received a replacement tube from Gene Malkoff- and also received the 2nd set of batteries from Mountain that had been lost out of the original package (Thanks, USPS!! NOT!)- and to my surprise, everything fit up properly!!! To be honest, I didn’t think the replacement tube would be any different. Not being able to easily remove the installed batteries from the original tube, I can’t accurately measure the tubes to compare- or the batteries, but heck- I’m just happy to have the whole thing together as it should be.
Again, my thanks to Gene Malkoff for sending me a replacement tube- and also to Mountain Electronics LLC for helping me resolve the lost battery issue promptly, even though it wasn’t their fault.
Merry Christmas, everyone!
… or Happy Holidays- you choose! 
Wes
This fit problem is nothing new and affects major brands too in addition to small producers like Malkoff. I have a new Thrunite that has a battery holder too short to hold most protected 18650 cells. Most are about 68.8mm long and the maximum length cells that will fit the battery holder as received are 68.3 to 68.4mm long.
My Jetstream RRT-3 has a body tube bored tight enough so that many protected 18650 cells when installed in it’s battery carrier make a package too large in diameter to fit in the flashlight body battery tube without using force. This is enough of a KNOWN PROBLEM that Orbtronic warns on their web site that there is a known fit problem with their 3400 mAh batteries fitting this light.
There needs to be a industry agreed to ANSI /ISO standard for battery dimensions for both unprotected and protected lithium ion batteries and then both battery and light makers need to stick to making items that meet the specs and can work with worst case battery dimensions.
Protected 18650’s are actually 19670’s and should be referred to as such.(according to Wikipedia) That is the standard. Unfortunately, no one does.
Even 19670 is not descriptive as many protected cells are considerably longer than 67 mm. 19690 or 19700 would be more real world numbers for protected cells and light makers engineers should be aware of this regardless of what protected cells actual dimensions are as long as their lights are intended to use them. Even then without ANSI/ISO standards there are no tolerances stated for these nominal dimensions and even items such as Panasonic’s unprotected 18650 cells are larger than 18mm diameter.
The whole situation is DUMB and IMO indicates either stupidity or a DGAS attitude on the part of some manufacturers.
How many reviewers complain if the cell rattles, though? So they try to make it 'just fit'. If they also sell cells under their name, they'll only worry about fitting those.
Find some Sanyo 16650. They fit in everything that’ll fit a 16340. Hank used to carry them, but I see he doesn’t have them anymore. They also have higher capacity than 17650.
Huh? 
I’m sure he meant that they will fit any 2 cell 16340 lights. I know that they fit in all of my old unbored Sure Fires. They are great cells, I have 6 of them, some protected some not.
Per my search results apparently Sanyo is now making a 2500 mAh version too. I am looking for a USA source.
Glad to hear you got it worked out. I have some Sony 17670’s that I pulled out of something years ago that have served well in situations like these.
I have many lights so a little reduced capacity in one light is not that big of a deal for me, but I agree it is frustrating to pay a lot for a new light and not be able to use it.