As far I know, a locked down and secure box can't be lockpicked. If the keyholder put key in a keyring, he can lock it down on top of box if he is friended to the house and only keyholder and house owner can click on key and unlock the box.
If you don't want house owner to have access, you need to give keyholder a vendor, to keep the key in ring. Now the house owner can't get the key but a house owner can always unlock and axe the box, so I better like the way with the key locked down, as with the vendor solusion, a thief can steal the key, when keyholder remove it from vendor to unlock the box, even when he can't use it unless he is friended too.