Ive an Ortlieb tail rack for this. Totally waterproof and a good shape for tents etc. Can sit in front of the top box or without it at all. Bungeed down for safety. Guess I should add a pacsafe for security
I bought a couple of cheap sailing type waterproof sacks from Lidl (thanks to a fellow tosser who informed us). Bungies to the back and is waterproof, cost me £4 I think
The better looking your solution, the more someone wants to nick it. Mines just a big black rubber looking sack. Approx 40lts IIRC.