So I bought a Sofirn SF31 (VG 15) during one of their recent irresistible Amazon deals. The light’s pretty good as is but was way to reasonable in terms of output and heat production so I had to fix it. The addition of a 4000K XP-L2 emitter on noctigon and MTN FET + 1 driver fixed the heck out of it but left an annoying 0.5mm gap between the bezel and the head. This is because the noctigon is just that much thicker than the stock MCPCB. Fortunately, the lens is a full 2mm thick allowing the purchase of a thinner lens to deal with the gap. What is needed is the 20mm by 1.5mm AR coated lens available at Mountain Electronics. Works great and no more gap.
Dunno the depth of the reflector, but how would a standard 20mm TIR lens fit in it? Generally, they’re a tad shorter than reflectors they replace (eg, in the S2+), but dunno by how much.