I have one of these 2 'D' cell flashlights, but I haven't done anything with it.
I also one that looks the same but is much shorter, and uses 4 AA batteries, in a battery holder that is the exact same size as a 'D' cell. I think this one would be good to mount an array of Nichia LED's, or a Luxeon Star.
I also have one of these flashlights in the AA size, in which I removed the head, and replaced it with a 1" copper pipe connector. Inside this new head was placed a MAX757 step up regulator I made, and a Luxeon Star w/o on a 1" diameter aluminum heat sink. I have some photos of it somehwere. I'll post them later.
I used black electricians tape to hold the new head on. It doesn't look that great, but it works fine. Lasts a couple of hours on 2 AA batteries at full brightness.
Because the inside of the stock head is plastic, with a stainless steel outside, ti doesn't work well for heat sinking the Luxeon, hence, I haven't done any other mods with it. This one modified flashlight I have, I gave it to my wife.
I did notice that the Lithium 123 fits inside this flashlight perfectly.