I use one of these
http://www.boot-bag.com/Can get a lot in it and it can double up as a holdall, not had any water ingress even in torrential rain
I second that, Boot Bag has loads of room and secures well to rack with plenty of straps.
We use a bootbag and loads of room. It also sits lower on our rack so unimpeded rear view. I use extra straps to fix it but those attached do a good job anyway.
The only mod I have done is to add poppers to hold down the flap that covers the zip.
