At the moment I have to use a landscape style air freshener which comes with string, which isn’t what they were really meant for.
Shops use such signs for “Open” or “Closed”; “Gone for Lunch”; “Back in Ten Minutes” etc. Holiday cottages use them with “Welcome to the Seaside” and similar messages. Homes might use them - “Home Sweet Home” etc.
If I understand you correctly, there is already a door hanger available. Is this what you were looking for?
That style door hanger works for doors with doorknobs, but not business-style doors.
Ahh, you’re so right @Jadendreamer13 . I didn’t have the right vision of what @NigelSutherland was looking for. 
No, the style I’m thinking of has a length of string from corner to corner which the plaque hangs from.
Funnily enough, I went to a pub (bar) nearby since I posted, and they had a few of these with various witty slogans printed on them, hanging on their walls!
Here’s the kind of sign I mean…
I will pass on the feedback, thank you!
Thank you, Fiona. 
