I did this a few different ways when paintballing.
In one guys truck, we set the tanks on their sides inside of a truck bed toolbox. We blocked them with sandbags so they would not move side to side and set another sandbag across the tanks so they could not move vertically.
The other way, I built a frame out of 2x4's and 2x6's. A box frame was built with 2x4's. 2, 2x6's had circular cuts in them to match the curves of the tanks. One of the 2x6's was placed at about 4 inches from the bottom and one about 4 inches below the curve on the top of the tank, were attached to the frame. A second set of 2x6 were used as clamps to secure the tanks to the frame. I carried 3 3000 cubic inch tanks in a box that was about 3 ft tall, 3 ft wide and 1 ft deep. The frame was tall enough to completely contain the tanks if it ever tipped over. The entire box was ratchet strapped to the bedsides of my truck.
I don't have any pictures, but I think you can get the idea. I don't have the box anymore either.
In one guys truck, we set the tanks on their sides inside of a truck bed toolbox. We blocked them with sandbags so they would not move side to side and set another sandbag across the tanks so they could not move vertically.
The other way, I built a frame out of 2x4's and 2x6's. A box frame was built with 2x4's. 2, 2x6's had circular cuts in them to match the curves of the tanks. One of the 2x6's was placed at about 4 inches from the bottom and one about 4 inches below the curve on the top of the tank, were attached to the frame. A second set of 2x6 were used as clamps to secure the tanks to the frame. I carried 3 3000 cubic inch tanks in a box that was about 3 ft tall, 3 ft wide and 1 ft deep. The frame was tall enough to completely contain the tanks if it ever tipped over. The entire box was ratchet strapped to the bedsides of my truck.
I don't have any pictures, but I think you can get the idea. I don't have the box anymore either.