just contact the owner. tell her your story and tell her firmly that you are not comfortable with the situation it is and you want to get the lock, put it in writing just in case. I don't think it a big deal. you are paying rent for your room. nobody has any business in your room. It is a pretty reasonable request. I don't think the owner has anything to lose but a free lock, and she'd keep the tenant happy. I'd like my tenant happy and comfortable, making the place like a home for themselves as long as they pay rent and take care of the place. I pay for the materials if tenant want to add something to the property. you need to provide landlord with the key.