I'm not into writing about males in any of my stories, but....
How about this, using some sort of a locking steel ring that she can secure tightly around his scrotum above his testes. It would have a chain long enough he could use the toilet and she wouldn't have to worry about cleaning up a mess when she got home. Take my word for it, no man would be able to pull away from that. The best types of bondage are the simple ones. Now that I've said it, it even scares me