I have an old Brooks rain cover, but got my wife a newer one (might be planet bike) but hers is nicer because it has more material and a draw string that nearly closes it up underneath the seat. Better for less water gunk getting thrown up if no fenders.
my Brooks one does fit well with elasticized edges, but more material would be nice.
I've been meaning to get myself one, but haven't gotten around to it all these years.
Overnight outside bike storage when camping, I put a plastic bag with a clip on top of my seat cover. If it's rains really heavily it's simple insurance.
clip or twist tie stops bag coming off in with high winds. Has happened.
I always too have a spare plastic bag in my panniers, again, easy insurance.