Wouldn’t it be easier to just put a few solder blobs on the outside of the driver? I built mine with a 1.2mm thick 20mm FET+1 and had no issue but I guess YMMV. The ring is ridiculously thick so i’ll leave up the pic even though it isn’t what you meant.
I did file down a little of the lower threads, but it had no effect on where the ring stopped rotating. Very strange. I may have to look at it closer.
Unfortunately I don’t have the 21700-C8F yet, so I can’t check myself.
I did this kind of filing several times with brass-pills (before the pill-less design took over) and it worked well to get the pill in deeper.
Of course you can do a comfy-style solder addon to the driver. Preferably the component side, as solder does not hold pressure well, especially not with a shearing force like from a retaining ring.
Btw, comfychair used a drill press to ‘mill’ these solder blobs flat. That was a classic.
Hey Sofirn next let give us C8F 26650 and the single emitter throwy version either 21700/26650 .
Why do you want 26650 when 21700 has almost the same performance but more compact?
You get longer run times, plus it feels better in the hand. The smaller 21700 is a bit awkward for me.
Then there is the SP33.
I talked Sofirn again. They told me that talk with the engineer and will think about the songle led 21700 version. I said that make it like C8F so same double switch just change the front end to single led.
Like ZozzV6 said, the old SP33 was a single emitter 26650 light. I’m not sure why they changed it to xhp50.2 with boost driver. Maybe low sales? It might have been better to have 2 seperate models.
That good I like the 2 switch with forward click tail of 21700 C8F , hope for bigger head (55-60 mm.) with XHP35 for better throw and please make the light switch to stay on as locator .
Now your asking for a completely different light. 
You want Sofirn to make a light like the thrunite catapult v6 or lumintop odl20c, but with 2 switches?
Yes that I mean Jason but with Sofirn price. 
I recently received 2 C8F 21700 hosts, along with a driver kindly sent by Sofirn, and today I found the time to assemble 1.
I used 2 pieces of 16AWG wire (about 4-5cm each), 3x XP-L2 V4 4500k 80CRI 3-step, and the Sofirn driver.
And I was really amazed by the result, I measured 17.8A max at the tailcap, with a 30T @ 4.2V
THANKS Sofirn.
That will be just under 5000 on-LED lumens, right? 
Anyone know if the 21700 host will show back up in aliexpress store?
From a test to ceiling bounce, I estimate about 4700-4800 lumens.
Not too shabby!
I modded up a C8F 21700 host with 3 351D 4000K 90 CRI's and also measured 17.8A at the tail! Mine does 4150 at start, 3750 at 30 secs, but that's not ANSI lumens, more like manufacturer lumens. Measured 33 kcd throw. I used 20 AWG LED wires (long to re-program), and swapped the tail bypass to 20 AWG.
I got one more 21700 host on the way (hope so), and got in 3 C8F 18650 hosts, 2 modded up.
One C8F with XP-L2 V6 5000K actually read 19.1 amps on a VTC5D cell, 5640-5050 lumens, 42.5 kcd.
I hope they get in more - doesn't sound or look good. They don't seem to sell the 18650 versions any longer as well.
I forgot to mention:
Sofirn said they will make a single led Version 21700 C8.
It will be the same design and driver and firmware and double switch as the triple but with single led with smooth reflector. The led options not clear yet but XP-L HI is definietly on the list.
That sound great … with bigger head for +150 kcd will be nice.
No other changes just single C8 reflector. I tb nk it will be the same as C8T and C8A