What's a workable low hassle privacy latch for the inside of an interior door (against the jamb)
Possibly a sliding vertical bolt, as more typically seen on the &quot;non-knob side&quot; of double doors?
Or hang the key to that deadbolt on the doorframe, since it's already there - change to a double-cylinder if it's not an egress path and the other side is a latch, or turn it around and don't need the key on this side. Or post a notice that it's not to be opened if closed unless there's an emergency... depends on the level of &quot;privacy&quot; required.
Presumably finding the key for the doorknob lock is a lost hope.
Life-safety-wise, consider that there are reasons (occupant passed out or otherwise unable to open from inside) that interior &quot;privacy locks&quot; (typical of bathroom/bedroom doors) have a means to be bypassed in an emergency, and that anything which blocks/locks an egress path is a bad idea!