I am doing exactly that and it sucks balls how precise it is…
Are we talking about the newer smooth C8 reflector? I've had awesome results doing just that. Perfectly centered and super tight hotspot.
Yes, new (shorter C8) SMO reflector.
Dimensions of XP-G2 emitter are 3.45x3.45 so the diagonale is 4.88mm while the side of XM-L2 emitter = size of centering ring sides are 5mm so there is a gap of 0.12mm.
Close enough for government work. 
How hard would it be to make them out of GITD material?
As an FYI my circular factory Nitecore MT-G2 spacer is 9.09mm ID 15.52-15.59mm OD x 1.01mm thick OR .3575 / .611 x.04 thick at the centering ring, the inner is thiner (probibly emitter thickness)
The ring is self centering type.
The inside of the reflector is 12.23-12.27mm ID or .48-.4825
+1 on the glow in the dark as an option, can you make a height adjustable centering ring? Or sell ultra thin shims for the final focusing tweak?
Hoop. If I Promise to only get in your way six times a day can l please come around and watch.
Isnt that Fasttech transperant XML to XP converter do the job? Havent played it alot, but i did it once with a XLP emitter….yet, the reflector was OP
Old-Lumens, thanks for all the specs. I have modeled them all and will add them to the order queue in post #1 here soon.
Can do but need dimensions provided.
Sure. For the life of me I can’t find my C8’s stock centering ring so I’ll need the specs as far as the od, id, and the thicknesses.
No can do. GITD paint is my suggestion. Get it from GlowInc.
Common paper has a thickness of .003” or so. The shims could be stickers or regular paper. I could have sticker company or printing company die cut these for me but they will probably be .50¢ per shim from what I’ve seen, if I get a couple hundred made. Alternatively, if you determine the optimal thickness you need through testing you could then have me make a centering ring to the exact size you need.
No can do. GITD paint is my suggestion. Get it from GlowInc.
GITD paint isn’t a great option, all the ones I’ve tried glow ‘spotty’ rather than as a full ‘block colour’ glow, this is the reason I made the flat type and didn’t use paint, plus the reflector will strip the paint off when it’s screwed down.
I have something in mind for a future project but I don’t know if it will work, time will tell.
Depending on the plastic you may be able to put one of my GITD gaskets underneath the centring ring i.e the clear white type as opposed to the white opaque type, will dim the glow but might work?
Updated OP with pics of current progress. The ring bosses for the MT-G2’s and XHP-70’s, the portion that goes into the reflector opening, is currently modeled at .030” tall. For smaller led’s I will probably do .02”. Feel free to specify the height of that if you wish.
Wow Hoop, thats great. I will certainly keep this in mind when prototyping.
Hoop - on the ring for the 26mm Maxtoch, you could go ahead and make the centering ring portion complete, on all four sides, as there is room on that one. Other than that, they look good to me, but others need to jump in and say what they think.
Ahh yes. Thanks O-L, will fix that.
Also, I’ve measured the MT-G2 at .352” square. .365” seems like a lot of clearance. Should it be smaller?
Might just be my incorrect measurements. I got .362" and went with .365", but I can go check again.
Edit: I measured one that was on a star and got .360" one side and .357" the other side. I went and measured one by itself and I got .353" one side and .350" the other side, so the difference has to be solder or flux or expansion from heating and soldering, (possible? probably not).
I would at least go with .360" myself, but you're the machinist and I am not the most knowledgeable on clearances and dimensions. My digital caliper is cheap, and not 100% reliable, but then again, neither am I.
Oh, the three I "gave specs for", are just suggestions, cause I haven't seen any other specs come out. I can't say they are the best or the right ones, just ideas that might work.
For sure, I appreciate it. I modeled the squares at .360”. I might have to add little notches to all of the LED squares to allow for the excess solder blobs… Or the end user could use a straight razor to cut off the excess solder. That works well in my experience.
Very cool to offer these things, exactly what I need right now. 
I’ll work out the dimensions and get back to you!
Any chance you could do black nylon instead of white?
Would everyone prefer black? It would be easiest to offer one color because I will cut these at the same time from a single sheet. I can do black though. Things made from thin sheeting like O-L’s .015” washers may not have a color choice, but I will look into it.