If the reason there is nothing lower than Q open right now is due to being on a popular travel date (such as around holidays), they may never open up inventory lower than K. Plat companion certs can be used with L/U/T/X/V fares -- Q/K are the next two highest classes above them. Also, some lower class fares have black out days around popular travel periods and holidays -- although these are typically X or V class fares.