The flashlight I am using is a Fenix L1D. I love this little bugger. It takes a single standard AA battery, and works well with rechargeables. It throws out a lot of light, but also has options to decrease the brightness level via twisting the end, increasing battery life. It also has a strobe and SOS mode but whatever.
I use this flashlight all the time because of it's convenient size, efficiency, and light output, but of course often need the use of both my hands, leaving the only other option of mouthing it. Often dripping drool into it. This easy as pie mod will create a dual purpose device.
You don't need much. Just the flashlight and some cord. I used just shy of 1.8m (6ft) of 550 paracord. This is plenty for a light this size, and you probably don't want to have one much bigger hanging off the side of your head.
The whipping length was fairly short (10 turns) so the thread is kept clear. Thinner cord will need to be longer.
The exact length of cord is not critical as it will be made adjustable.
Whip ItDouble the cord along the flashlight, creating a loop. It is not folded exactly in half - one end is long - this is the line that is wrapped around the cylinder.
Hold at the point where the whipping will start and fold the long line over the short one, clamping it in place. -->
Whip It Good<-- Wrap around again and continue. Whipping needs to be TIGHT to work so pull that sucker HARD.
Continue until you almost reach the end, leaving a small area free. -->
If you whip it right up to the end, a loop may slip off which will loosen the whole works, leaving you with a lovely birdsnest. For this reason, stop wrapping at about 10mm from the edge.
Shape It Up
<- When you have wrapped enough loops, take the loose end and feed it through the loop.
Now pull on the short line that lays under the wraps, and pull it tight. -->
Get It Straight
If you are not familiar with whipping, this is a pretty standard technique and can be used for many other purposes such as securing two or more poles together, or making comfortable handles for tools.
<-- Take one end of the cord and tie a knot in this fashion. This image shows the lay of the knot but you will want to tie it tighter than this before you try to sinch it up.
When tightened, it will slide along the other line and not slip under tension. Tie a knot on the other line to stop it from slipping. I used a figure-8 knot but a granny knot will do.
If you whipped it good, it will never come loose and you will never lose a valuable item such as a 90 fucking dollar flashlight ever.