Home Depot or any hardware store is your friend when it comes to mounting solutions. Save your cash and DIY.

I have 2 mounts with bolts, washers and the locking nuts and they have held for 2+ years no problem.
I carry 2 two gallon gas Rotopax and 2 two gallon water containers when we hit the trails.
That's 4 gallons gas and 4 gallons water for a total of 8 gallons. It's pretty heavy and these have held up to a lot of wheeling.

The pics below are the base mounts, then I put the extensions on the Rotopax mounts to fit all the containers.



View from under the rack. Suggestion, to mount these get a long ratcheting wrench .. like this one: Husky 100-Position Double Ratcheting Wrench Set SAE (6-Piece)-H100DRW6PCSAE - The Home Depot
so you can reach the nuts on the bottom side and then just crank down on them. Unless you have really small hands it's impossible to work under the rack between the roof of the 4runner and the bottom of the rack.



This dude does an ok job of demonstrating some of the extension and mounting solutions.
YouTube


Edit:
Not the greatest angle to see the mounts, but you can get an idea of how far 2 packs on top of each other sticks up above the rack.