All that's needed is a bicycle tube, preferably not a new one, reduce reuse recycle n all that, and a scissors.
Just follow the images. I'll get a full ible up for this and a few other ideas in a week or three.
It's not the cleanest but it is very effective, I'm currently using one as a wrist lanyard for when I'm in the pool and have to keep a key to my gym locker.
Now time for a sausage roll.......