First, the cheep and low tech....
Purchased some of that knobby stuff that you can use to line kitchen drawers and put that down in the bed of the truck and placed a few bricks on each side.
Placed the tank against the side of the truck and then put my weights on the other side of the tank
Or you can find a scrap 2x8 at a local building site and use a saw to cut a vee notch (or a semi circle if you got the talent)
Finally, the shop do sell some items (some made from a foam material) that will keep your tanks from rolling about.