The Hellfighter draws a substantial amount of current, more than the OEM cigarette plug was designed to handle in fact.
Here's a basic schematic if you're trying to make a straight cable. The main military connector's pin's are labeled and I've used that same lettering. DC+ comes in on pin B, goes through the handle switch, out through pin A, then back to pin C and to the ring terminal on the head. DC- goes clean through pin E to the center terminal on the head. The reason for the wonky circuit through A/C is to allow for a cable-mounted remote switch the original applications had, like on the spade grips of an M2 machine gun.
You're going to need the 5-pin military connector (the model # escapes me but you can Google--it's available from commercial electronics suppliers) for the end of the cable. You'll also need a stout cigarette plug (not a great means of supplying power--I'd suggest using a different power source,) some good quality rubber SJ cable (IIRC 16 guage,) and misc supplies. Adhesive-lined heat shrink tube works really well to clean up the cable ends. When you're soldering the connector you'll probably find the original rubber pin insulator gets in the way. I'd suggest skipping that and instead potting the solder joints in hot glue or silicone seal once you're done. Other than that it's a simple arrangement for someone with soldering skills.
Note on the cigarette plug if you need to go that route: Nobody makes "good" ones, or at least didn't when I had a Hellfighter. Surefire used some flimsy underrated one from Partsexpress. I found a better one from a marine supplier and used that. But in both cases the only way to make it work was to gut the connector and run the SJ cable directly to the teriminals. Otherwise the tiny wires inside the plug would be insufficient for the load. So if you don't have to use it in a car, use a different power connection. I did one with alligator clips and another with RC battery terminals, but you could use anything you wanted.