score:4
This article from the CNBC websites mentions exactly what you need: a day room at Frankfurt airport. Turns out there are various day room brokers out there, as there is an obvious market for this service: travellers and businessmen on the road.
A rapid search revealed that by far the widest number of options for a day room at FRA seem to be listed on Between9and5. Below is a screenshot of the room availability for the 17th of July as of today: