I have used cheap shower caps as helmet covers
I have a cpl saddle covers. mostly only cover them when the bike is outside w/o me on it, or when it rides on the car in poor weather
some are better than others. meaning some fit better, some are more water proof, some are OK to ride on. if I wanted to cover a bike saddle & keep the cover on 24/7 then I would buy a diff. saddle
this one is very good
Brooks England Nylon Saddle Rain Cover