The Rotopax system is really versatile. They make mounts, extensions, and a lockable mechanism to secure the cans. It's up to you to decide where and how you want to implement the mounting.
I don't have a proper write up on this mod, but here's some pictures to give you an idea for how to mount the Rotopax cans to your crossbars.
One of the great features about the cans is that you can inter-lock them end to end. I chose not to mount them this way to allow for easier access. I use the red cans for fuel and the white cans for water.
Each lock comes with a key set. Just turn the handle to tighten and secure the can. To remove, turn the key, pull the lock out of the handle then turn to loosen. The can is easily lifted for removal.
Notice there is some deformation of the fuel cans due to pressure and heat. I first noticed this happen when I was in Colorado...I put fuel in the cans in Texas. Several weeks of 100+ F weather may have contributed to the deformation, but to date there's been no leakage. The Rotopax seals very well. At the time the pictures were taken the cans were empty of fuel.
Here you see one can secured to the mount. Above the can is the extension added to allow a second can to be mounted. The handle with a lock comes with the original mount.
And now with no cans. You can now see the mount, the four bolts that are holding the mount to the grill I fashioned, and how I bolted the grill to the crossbars. I painted the bolts, nuts and washers to help them blend in and not be so noticable.
This setup allows the grill to flex a little, but the mount keeps the cans secured to the crossbars. I don't know what advantage this gives, if any, but so far it has worked without failure. I've had this on my FJ continuously since the beginning of July and it's survived a trip to the Summit, trails and daily driving.
I hope this helps and gives you ideas for how you want to mount your Rotopax cans. I bought all my RotoPax accessories from Jacque at TRDparts4U.com. Give them a call...they stock all that I've shown you and more.