Definitely the D.I.Y. Illuminated tailcap by pilotdog68.
I am still a bit crap at soldering, so in the end I chickened out and bought the BLF X5 that already had this mod pre-installed, but it’s something that I one day hope to build in most of my lights. I don’t mind the constant drain, I don’t get to use my flashlights often enough anyway, so why not keep the batteries busy? 