I rent a room in a private home and cannot change the doorknob, but I would like to be able to secure my room from others staying in the home. Is there a way to secure an outward opening interior door that does not have a lock?
I would search Amazon for "portable door locks" and add a long-shackle padlock to this type of device
You need the padlock because the majority is accessible from the outside. The whole thing will obviously be visible and not resistant to detrmined attack.
The lower photo shows how it locks into the door frame, preventing the door opening in its usual direction.
This works because an inward-opening door that an occupant wants to secure while asleep is essentially the same case as an outward opening door you want secured when you are elsewhere. The added padlock prevents passers opening the lock.
Make a request to the home owner / lease holder (who ever it is that you rent from) - that you be allowed to put your own door knob with Lock on the door - they can keep their matching door knob somewhere else in the house. Generally two screws accomplish the change out. YOU SHOULD BE THE ONE TO DO IT and NO KEYS to anyone else.
This is a fair and reasonable request and I think it is a small thing since you are paying the rent for the room. If the owner will not allow this you should look for another room elsewhere to rent - something is probably not right.
I would also look for something like an iSmart alarm so you know if by some means anyone has entered the room or opened the exterior window while you were not home.
Further when renting such a room as this with access for whomever - check for hidden devices and cameras. I assume the room also has it's own full bath.
Of course if you are intent on staying there regardless of what the owner says about changing the door to a lockable door knob - get yourself an iSmart Alarm so you can see when and how many times entry is being made and ask for what purpose?