You could do a sst40 or xhp50.2 3v if you want to make a lot of light. Don’t need a boost driver.
Side comment (no offense meant to Convoy): Lumintop EDC05C is smaller than S2+ and supports 14500 battery only — it uses XHP35-Hi and thus a boost driver, so it’s possible to have such a small boost driver, but I don’t think they will sell the driver only… not sure what’s the size of the driver on the EDC05C.
… a lot of low CRI light, which is a no for some.
There are other benefits of using a boost (or buck-boost) driver, and that is full regulation; i.e. output to the emitter is independent of battery voltage or state of charge.
The sst40 tints are a bit cool for this project but the xhp50.2 3000k looks very interesting. I don’t see parts for the s2+ though so guessing a level of wizardry above my skill may be involved?
https://convoy.aliexpress.com/store/330416/search?SearchText=xhp50.2
I don’t think Simon has the 3V 50.2 - cutter.com.au does though, but they only come in ~5000K or higher.
(3b0)
I haven’t seen a 17mm XHP50.2 driver though off the top of my head (6V).
As others are saying “3V” XHP50.2 emitters only come in above 5000K colour temperatures, and no CRI choices (KD has an 1C for sale, that's all I know).
It is possible to equip a 6V XHP50x in an S2+, but as others have said it will either require a ∅17mm boost driver (like a custom built GXB172) or a ∅20mm boost driver (H1-A) soldered to the pill with probably some filing inside (jensen567 did this, check: Buck and Boost Drivers, Testing, Modding, and Discussion (Pic Heavy)). I was once going to make a build like that, but decided that my H1-A (with 2R2 inductor; they :facepalm: downgraded it afterwards) would go better in an S21A (with ∅20mm driver cavity) or similar.
@Simon Mao
I’d love to see M21C with USB-C, I’ll keep waiting! 
3x21700 with USB-C sounds great, is this flashlight already available in you shop? What’s the name of this flashlight? Can You give a little bit more details? What LEDs are used?
Thomas
Yeah, I’d definitely “need” usb charging with any multi-cell (parallel) light. It’s too much of a pain to have to drag out the Opus to charge all the cells at once.
That’s why I pretty much mothballed my Q8s with the nicer/warmer tint and floodier beam, and have my DC7 with a somewhat yickier beam instead (cooler, throwier). And the Q8’s annoying O-ring that just wants to escape, makes it even more of a chore to have to open it up then reclose it.
All the new incarnations of the Q8, with ’351s and Andouille and all sorts of bells’n’whistles, if they don’t include built-in charging, I can’t justify getting one.
Anyway, point is, built-in charging for parallelled multicell lights would be a huge plus in my book, and almost a dealbreaker if not included.
3*21700 and 3*sst40
It will take a long time to put into mass production, MCPCB needs molds,Driver outputs linear constant current with temperature control,I can’t give much information yet,
That’s gonna be a chonk of a light, but I love it.
Better with 3*xhp70.2 
M21C with XHP70.2 and USB-C would be amazing Simon.
I am interested.
with 4x18 single emitter reflector. I’d be happy with just the driver installed, as a host so only to add SBT90.2.
I am interested in the 4X18A with SBT90.2.
+1
… but 25A driver would be awesome!
I’m not sure a video is helpful or even a possible way to show this. It often occurs between uses of the light. So usually after hours or days of being turned off. I’ll go check the lights they are installed in now and see if/how many have forgotten their group… I’ll PM if there is anything to show. Thanks for your attention to this!
Best wishes!
Jared
+1
XHP70.2 have low efficiency comparing to XHP50.2.
S21A with delensed SBT90.2 is better option... i hope, still building
To my knowledge it’s exactly the opposite.
Example tests:
XHP70.2 is more efficient at every current.
The emitter is also near the cost of the entire light.
Actually emitter cost double the host :/
But the joy is priceless