You could make something that would work fairly easily with a basic lathe and mill. It wouldn't have the internal splines, but those really aren't needed. The lather models just use two nuts and a washer. As you found you will most likely have to purchase the entire assembly to get the single part you need. IMHO you have the more critical part, the top piece just locks it in place. If you really want precision I would convert it with a cheap digital caliper.