Does your motor already have the prop adapter similar to what Speed posted? It looks like the BL15 has an integral drive hub, so all you'd need is a spinner nut and washer. If you have a means of attaching your prop and the only problem is the hole size, there's nothing wrong with making a bushing. There is no side stress on the hole once you've tightened the prop nut. All the hole really does is locate the prop in the right place for balance. I've even wrapped tape around the shaft before to make that work. It didn't cause any problems at all.