Originally Posted by porter

Not sure what you intend to use it for - but I have a cheap Ortlieb waterproof sack with a roll top closure that I lay on top of my rack. It cost something like $20-30 australian (means it must be practically free in the rest of the world). At the time of this picture it was used for carrying my tent and sleeping mat (rolled around the tent). I tie it down with one strap and hook the front roll closure around the seat tube. Keeps everything neat, tidy and dry.