I am currently renting an apartment with a few flatmates, but it seems that the keys to the bathroom door have been lost. The owner of the apartment is not open to modifications.
We are trying to find a way to prevent embarrassing accidents on opening the door when someone else is in the bathroom. We are not very concerned with the safety aspect of the locking mechanism or whether it actually even locks the door; a way to prevent such accidents would suffice.
I found Temporary lock for a door, but it seems that the use-case of the asker is different in that question. Also, the suggested solution, which is a door knob lock-out device, is not readily available where I live, and would be expensive to order from outside.
This is how the locking mechanism looks like:
I believe the lost key looks something like this: