You need a small sheet of aluminium, about 1mm thick. Using a piece of paper as a template work out the dimensions that it should be. If you are making it as a bracelet wrap the paper around your wrist and mark how long it has to be so that it can easily be put on and taken off. If you are making it as a serviette holder do the same thing, it may help if you have a look at a commercial one so that you can copy the dimensions. Make it a few centimeters longer than you measured so you can clamp it in a vise for ease of bending, this can later be cut off. It is always better to make it too long than too short. I made mine to the dimesions in the picture with a lenght L that I determined later. Then mark the template onto the aluminium. I cut mine using a rotary tool.