One other way to rub the edges smooth is to use a piece of heavy canvas of hessian after having wet the edges first. I have tried Rick's idea using an electric screwdriver with a wheel and it is by far the fastest way to do the job.
Rick, what do you find is suitable width for banjo and mandolin straps? I usually do banjo between 1. 3/4 and 2.1/2 and mandolin between 1.1/4 and 1.3/4. Many people ask what is the right length; in my experience the strap should be comfortable and hold the instrument in the same position weather seated or standing (I can't undrestand how "rock musicians" can physically play the guitar when it is suspended around their knees)As Rick said I would be interested in learning any new ideas from anyone who does leather carving. Remember a carved strap is for life, not just for Christmas so choose your design well, you have to live with it
Cheers Bill