Sorry for being a bit touchy.
I in no way consider myself an expert (hence this thread) but have tried to educate myself in the safe way to use these cells over the 3-4 years I have been using them ie good quality reputable cells & chargers, matched cells when using multiples, not over discharging, checking capacity & voltages etc etc.
I am a member of CPF also & have read & digested the sticky threads over there.
I started off (like a lot of people I expect) with Ultrafire cells & chargers but quickly learned that their claims were false & that they were potentially dangerous so changed over to quality products from reputable suppliers.
Light wise though apart from an Olight H35 wave & Fenix H11 headlamps my lights (headlamps & flashlights are cheap Chinese ones).
Although not delivering the claimed outputs or having particularly great build qualities I think that by using quality cells & chargers, checking voltages etc that they are pretty safe in my useage.
I have fancied modding a light for a while now, hence the question.
The use for these lights would be attached to the head of a fishing rod rest to illuminate the rod tips in the dark.
I presently use a pair of the single 18650 cell Ultrafire XM-L T6 lights for this (the zoom is handy as I can focus the beam on the top 1.5-2ft of the rod tip to register bites) & they work well but I would like the longer run time of the 2 cell light.
As they are so cheap I thought to buy 3, 2 to use stock on the rod rest & 1 to potentially try modding.
2 18650 in series could extend the runtime but not the efficiency of the circuit, it would be a better choice to swap the battery when empty than having both the same time. Less weight on the flashlight too.
These pre build zoomie lights are not the best platform to start modding, I suggest convoy C8, S2+, eagleye X6, yezl y3, Luckysun D80, courui D01, or uniquefire zoomie series. All these have been extensively modded by hundreds of members in so many ways.
We can help you choose what and how to mod, just tell us what are you after, brightness, reach, runtime, ect?
Modding would really be just for me to play around so I could learn a bit about modding.
I know that the light I have linked to is not suitable for modding to be a good thrower (I don”t think ?) but ultimately after I have done some successful mods I would like to build a good budget thrower as my headlamps & flashlights are all floody.
With this in mind I am still tempted to buy 2 of these zoomable lights for my fishing use as they should have longer run times than the single cell versions I already have.
Can you suggest a budget host light that is easily & cheaply modable into a good thrower, if such a thing exists ?
You said you got a bunch of cheap chinese torches, did you check them for moddability?
Maybe you got 1 with nice heatsinking and easy to take apart to start with.
I have half a dozen Chinese headlamps that are copies of Led Lenser H7”s (single 18650) & HR14”s (2 x 18650 in parallel), the 2 previously mentioned single 18650 zoomable Ultrafire XM-L T6”s, 6 zoomable 1 x AA SK68 clones like these http://www.ebay.co.uk/itm/14500-Mini-Portable-SK68-CREE-XM-Q5-3-Mode-Zoomable-600LM-LED-Flashlight-Torch-/291668135783?hash=item43e8c68f67:g:DjEAAOSwbdpWW-5h & 4 unknown 3 x AAA 3 watt Cree that look like these Laser 5055 CD Aluminium Torch With 3w CREE Bulb for sale online | eBay but only cost about £2.50 each when I bought them.
Unfortunately I wouldn”t know myself whether any are good candidates for modding.
Well decided to forego any modding myself for now & put my name down on the group buy of the CPFi/BLF Cometa WARNING: Cometa – read/fix BEFORE inserting battery 
Great choice! Be careful not to set your fishing rod on fire with that thing! :D
It won”t be used for lighting up my rod tips for that I will continue with the cheap Chinese single 18650 cell lights (claimed to be xm-l t6 but are probably not).
They only have to “throw” about 8-10ft & even they can do that ok 
They are in fact exactly the same as the “infamous” G700 best light in the world flashlight. 
I know somebody who has a torch that looks very like that, bought before the fake Cree lottery started. It’s quite heavy so it has a decent amount of aluminium in it. It could be the same or it could have been “value engineered” with a nice fake Cree LED and some thinner walled tubing, you won’t know unless you order one.
I have seen lights of that style listed, but didn’t need another 2 x 18650. For that price, especially on Ebay, it is probably not a real Cree XM-L LED. I would look for a similar model at Fasttech or buy another Z5.
At the end of 2013 I bought three Cree XML zoomable torches from Ebay for around that price (the cheapest cost £4.72) and they all have genuine Cree LEDs in them. There are torches claiming to contain Cree XML LEDs for £1.53 delivered so this isn’t as cheap as the cheapest. I’d say it was possible in the recent past to get genuine Cree LEDs for that money, today its still possible but I wouldn’t bet money this being genuine!
I’d stay away from the flashlight described in the OP. I like many have at times been caught out with the bargain flashlight , only to find it a disappointing purchase. The specs may seem ok, but we’re seeing fake Cree Leds apear and poor drivers. Spend a little more and buy a known brand product or as suggested by others in this forum in the earlier posts.