The tank portion is fairly straight forward. I took 4 2-liter bottles, cut the bottoms off of two, inserted one into each of the cut off bottles and then taped them together. I zip tied the 4 bottles together and then painted.
This is the base of the backpack to hold the tanks. I bought a kids backpack from Goodwill and sliced the back off of it. I also bought a basket just for the frame and sliced the basket the basket of of it. I duct taped the parts together. The duct tape will be hidden by the tanks themselves.
Here's the finished product (not attached to the paint roller yet).
I'm not really satisfied with how it came out. I'm probably going to scrap it and start over with a different design. In the meantime, I'm going to start working on a prop for my daughter for Momocon.