TH groove knots
One of those inexpensive 1 AA battery or 14500 rechargeable lithium cell LED zoomable flashlights that I'd bought a couple of years ago(probably ebay <$4), where the rechargeable battery cost twice as much as the light, lol.
I used some red 0.9mm cord to tie the three lead/part turks head knots in the grooves of the flashlight body. I didn't count the bights, just kept adding them until it got too tight to do any more...
The light doesn't have a lanyard hole, but the pocket clip has cutouts where I've added paracord for fingerloops and wrist lanyards on occasion.
The 14500 batteries that I have are a very tight fit inside the body of this light, where they go in easy, but are difficult to get back out, so I prefer to just use a regular AA battery in this one even if it does mean it's not as bright.
Those batteries can vary slight in size from one manufacturer to another just enough so that they might work in one light and not another. I have an EagleTac and Nitecore version of the 14500 which are also slightly too big for another light that is also supposed to use them (Sunwayman R10A) and have seen mention by others on CandlePowerForums that some batteries fit and others do not.