Locks that work

I hear what you're saying, and thanks for the reply, but it's a rented property and it doesn't seem worth the hassle and expense of getting it done when I'll probably be moving on in a couple of years or so.

You know your circumstances, of course, but even good ground anchors are not expensive, and fixing one is just a matter of drilling a couple of holes, then screwing in the supplied security bolts. It's nreally ot a big deal. If you're going to be there for a couple of years, it would be well worth it. Even a couple of months, maybe.