I used to use bungee cords until my mate had an accident and the ladders came off his van and straight through the other cars windscreen luckily no one was hurt .
yes i know there not everyone's cup of tea, i use the heavy duty ones, 2 per ladder. never had a problem with them, if i rock my ladders the van rocks as well. there not going anywhere.
a chap i know uses really cheap ones from the pound shop, doesn't replace them until they start to fray, asking for trouble
tony