The principle of operation of this craft is not new, but the cost of factory analogues and their material makes you think about home-made manufacture. Craft is quite simple, but requires care and accuracy of measurements.
To create a template, you will need two small strips of thick metal, in which you need to make holes and connect by welding at a right angle. The diameter of the holes and the distance can be made based on the required dimensions.
Use the craft if it is not large, conveniently using a clamp. If desired, one of the parts of the template can be made longer for convenience.