I recently bought some Think Tank Red Whips. They're made for cable management, but I mainly wanted something to add to my tripods; I use it to strap down my camera strap and shutter release when it's windy to prevent them from swinging around and causing vibrations.
Would you know if Red Whips are strong enough to hold up a flash? There are times when I'd like to put flashes in places where I can't use a light stand and no flat surface for the flash to stand on but plenty of bars/branches/whatever to hang from.
I just tried strapping a Yonguo 560 II with batteries to a tripod leg and it was fine, even with some up & down shaking. But there are probably some caveats. I've added foam covers to my tripod legs, which gives it some friction. I'm not sure how it would do if you strapped it to a vertical smooth metal bar, for example, I could see a larger, heavier flash slide down a pole. Also, I put both the leg and flash THROUGH the loop before tightening, but you won't always have that option, and one of the whips isn't long enough to loop around a thicker object and a flash; in that case, you'll need to hook 2 whips together (the package I bought has 10). Not a big deal, they're so small and light you won't notice the weight of a pack in your bags.