I usually carry them in the rocket box if I'm going to stop anywhere, for security. Or stuff them in a boat.
If you go horizontal to the ladder, you'll need another support post forward of the ladder. Then you could use ski clamps, or just ratchet straps. Or you could strap them vertical to the ladder, if you add something to the bottom to support them. A rectangle trash can zip-tied to the ladder would probably work.
2000 E450 dually V10 wagon