I have one mounted to my roof rack, I made two mounts out of a long 1/2" bolt and a piece of flat plate. It is mounted to the rack with little U bolts. The big 1/2" bolt sticks through the holes in the jack, and there is a nut/washer lightly screwed on, and I drilled a hole in the end of the bolt for a hitch pin so that the nuts don't work loose.
I did something similar on the other truck in the background, but I used carriage bolts up through the wood bed and just some big wing nuts to hold it in place. I wasn't as concerned with that one because it's just kinda sitting on the flatbed. You can kinda see it in the middle of the ladder.