In my own, personal, flashes (Not the "up kilt" type ), I use elastic measured off going around the leg and cut to size. I then use the clip designed for the bra of a female bikini:

I then make the flashes themselves and slid them on.

The advantage is it lies very flat on the leg. It is also VERY easy to make. I have made 5 sets of them (largely when I can't find my other ones) and they take only a few minutes to make.
The disadvantage is that they are not adjustable. If made too long, they can always be shortened by snipping and making a new loop for the hook. If too short, trash/give away and try again.