I've got a small Oxford first time one that works ok, has a flap that goes under the seat to keep it secure.
Also used a larger recently, cheapo called "slapper". worked ok too, though didn't have the underseat straps.
One tip - some of the bungees were rubbing against the paintwork, so I used the old trick of a piece of gaffer tape on the paintwork. Thing is even though I stuck the gaffer tape to my jeans and pulled a couple of times to reduce the amount of sticky stuff left, when I removed it after the tour there were still loads glue residue on the paintwork - which I stupidly scrubbed too hard and ended up scratching and tarnishing the paint. So, while the gaffer tape idea may be useful, be very careful how you remove the residue if there is any left. People since have said I should have used nail varnish remover, petrol or white spirit instead of scrubbing hard, but its too late now!!
Plus - get yourself some decent bungees, not the cheap ones from the pound shop!! A few short ones were just a useful as the longer ones!