I got a couple of them on Ebay. For the front and back doors. I also installed a chain lock on teh front door (when I was privately renting), and locks on the windows that only allow them to be opened a couple of inches when I was in private lets.
Obviously, when I was out it was a different story.
But I started doing this after a scary thing happened to a work colleague.
She moved to Edinburgh from a northern English city. She'd never lived in Scotland, so didn't know many folks and just got a wee flat in The Old Town.
A few weeks later she said something was wrong with it. She started to think it was haunted because small things would be moved from where she left them, doors open that she'd left closed, that sort of thing.
Well, I believe in that sort of thing so I gave her my mobile number and told her to ring any time if anything bad happened.
Got a call at 1AM one night, she was hysterical and roaming about in the High Street, wanted a lift as she was too afraid to get in a taxi with a stranger.
So DH went over and collected her.
She'd woken some time after midnight with a horrible feeling that someone was in the flat.
Went into the hall and the front door was standing wide open!
The fucking landlord was in the flat!
She threw something at him and screamed and he ran out.
The next day we rounded up a group of students to go with her to collect her belongings (another colleague had space to let her store her stuff until she found a new place to live) and leave.