I got one of these Master Lock portable key safes from safeoptions.co.uk, I am using it to lock the shed, and also to store the keys to the barn. It would be hard to fix a standard external key safe to the barns stone walls, but this portable key safe locks onto the padlock staple, just like a normal padlock. The dial combination can be changed easily by pressing a small button inside the lock.
The MasterLock Portable Key Safe is ideal for stone buildings where it may be difficult to attach a standard external key safe.
The padlock has a rubber coating which has protected it not only from rain, but also from the half bucket of mortar that fell on it, I was concerned that the mortar may have penetrated the lock and fixed it permanently in the locked position! Fortunatley the protective cover did it’s job and lock still operates perfectly.