I wear a locking heart bracelet that my Dom bought off Etsy for me. Its on a small chain and the heart is about the size of a fingernail. I never take it off but it can come off, but you can buy ones that once on would take two people to take offI have been contemplating what to use as a collar. I wanted something discrete that she could wear anywhere so that it never needed to be removed. But I also wanted something with a lock but that seemed to rule out discrete.
Today on Etsy I found a stainless steel chain with a functioning heart shaped pad lock...