In my opinion, the main issue with the bungees mostly used is safety. A bungee with a hook on the end can cost you your eyesight.
I use bungee cord, what sailors use to call shock cord (a cut piece, no hardware), for tying items to my bicycle rack; I think the same procedure would work for a ladder on a rack. The knot I use is a fairly new one, the Gleipnir, whare I mostly tie with two splayed loops, rather than around the whole thing. I.e. one loop around the rack one side of the ladder, the other loop on the other side and the TurNip in the middle. The Gleipnir is a very simple knot that will hold the two ends in one single turn http://igkt.net/sm/index.php?topic=1449.0. If you’d rather use cord, the knot serves well also in other materials.