Maybe we could just have someone put up a sign? [Someone who could make things permanent in liminal space, at any rate - unlike Armin.] There are closer bathrooms, so it's not too likely someone will just wander up here anyway...

[But he's still nervous about it. He shrugs uncomfortably, feeling a sympathetic twinge from the scar on his right shoulder - a memento of the first time this sign appeared to them.]