The tare weight of a 2-gal. Rotopax container is 3.8 lbs. A 2-gal. Rotopax container filled with gasoline will weigh 16.4 lbs.; filled with water it will weigh 20.5 lbs. Two 2-gal. water cans will weigh 41 lbs. Two 2-gal. water + two 2-gal. gasoline cans will weigh almost 74 lbs.
Whether or not your S*ittybilt tire carrier will be up to the task is for you to determine, but if you want bolt-on can carriers you might investigate the can mounts offered by Rock Hard 4x4, which come pre-drilled for Rotopax mounts. These fit many brands of tire carriers made from 2"x2" square tube with 45* lateral supports.
http://www.rockhard4x4.com/product_p/rh-2003-lt.htm
From the Rock Hard 4x4 website:
My jeep with Rock Hard 4x4 rear bumper and tire carrier with can mounts, 20 L Wavian Nato-style fuel can and ProQuipp 20 L water can from Australia:
I would not use Rotopax 3 gal. or 4 gal. containers. The three gallon size won't nest with anything else and the 4 gal. size is heavy and unwieldy. The 2 gal. size is easily lifted with one hand.
You might be able to strap down your Rotopax containers in the area behind the rear seat and not need any bracket. The 2 gal. can measures 17.7 x 13.3 x 3.2 inches.
BTW - I have been using Rotopax containers for several years. They are a great space saving option, but in the end I returned to standard size 5 gal./20 L containers even though they are heavier. With my current set-up I use a Super Siphon for transferring gasoline to my jeep's fuel tank without having to remove the jerry can from the can mount and I have a spigot on the water can for gravity-fed running water.
Here is a photo of my Rotopax cans in use on my "overlanding" vehicle, two 2-gal. gas cans with two 2-gal. water cans underneath them. [Photo taken on the Fins and Things trail at Moab]:
Here is another shot of the same vehicle loaded with gear for a 10-day trip. Note the Rotopax water can, which is occupying a convenient space without the necessity for a fixed can mount.
