Bungee cords have too many disadvantages. You can be careful and still have problems. Straps are safer. They're useful on ferries and I've a tied a few together to put my food up.

I wrap the strap one time around the rack tubing (see photo) and the extra length around the seat post with enough give to strap my tent down. I never have to remove it and it won't fall in the wheel. At worse, it will rub on the tire if I forget to close the clip and ride without a load, but it won't catch in the spokes.