I don’t like the bungee idea, still too flexible under the air flow pressure encountered when driving.
Stick to a ratchet style tie down or rope.
When laying the frame on those canoe cross bars, tie it tight ( rope ) to the to the bars where it crosses them then use ratchets or rope at the corners as well.
You’ve likely seen many “funniest home videos” etc showing people toting mattresses and plywood and such…
don’t do that ! 