I bought one of these at the weekend.
http://www.gooutdoors.co.uk/hi-gear-lugga-cargo-90-travel-pack-p143464
This is to fit on my Maxda stainless boot lid rack. The bag is great, it is PVC coated and appears to be very well made. For £32 I think its a real bargain. The guy that served me said that others had bought them for just the same reason. A flap covers the top zipped area to help with keeping water out. To strap it to the rack, I have also bought a couple of these to hold it on.
http://www.fasty.co.uk/straps/fasty_straps_over_centre.php
Will be testing it all out in a few weeks on a trip to Wales