If you're talking the typical aluminum sliding patio door, I've always heard that the easiest way into these is to just lift them off the track. One way to make this difficult is to screw a shim to the guide above the doors.

Obviously you need to take a look and see how feasible this is, and how thick a shim you'd need.

There is other hardware out there as well to lock these doors--but I'd think you still need to prevent them from being lifted out. And it could be an inexpensive and easy thing to do.

Good luck,
-k2 in CO

I got to thinking after my previous response that I failed to mention that old stand-by, the cut-off broomstick--to help keep the door from being forced open.

But even with that one, you'll still want to prevent the door from being lifted out. Between these two, you have a reasonable inexpensive "better than nothing" solution.

Best regards,
-k2

Here's to a smooth closing and to getting the best possible rate!

Cheers,
-k2 in CO.