I need to install a lockbox in a refrigerator to keep a medicine locked up. What is the best way to do that?

I had the same problem with my mother who was unable to remember how often she was taking pain medications. The solution was a locking "cash box" from a local office supply store.

The medications were in the box, which fit nicely into the refrigerator and was easily removed. I kept one key and the other key was in a "realtor" style lock box on the front door that the visiting nurse had the combination to. So the nurse gave her the meds in the morning and I gave them to her in the evening.

Obviously this is not highly secure but for an elderly person it was secure enough.