Ran across this. Sounds like an interesting idea to use as the enclosure for the water bays. In our case, the water tanks (fresh, grey and black) are strapped to the bus and we need an insulated box to protect the tanks from damage. This method sounds like a winner... lightweight and omits the heavy plywood we were planning on using. Gotta show this to David. I wonder if the roll on truck bed liner would work the same. Build the box, coat it with the liner, strap it up in place (since it's lightweight we could use that plastic plumber strap), spray canned foam in the remaining openings then spray canned truck bed liner over the dried canned foam to protect it.
We could do this for the water bays, fluid heater for the hydronic heat (no insulation or outer case on old tank) generator box, air compressor box. Anything heavy we have will be mounted to the bus itself and then boxed in. If the boxes are durable enough, we may use the same method to build a storage bay and add a piece of plywood to the floor of the box. Plywood & lumber just keeps going up. I'm upping the prices on them every week. Those are the departments I have had the most price changes in recently. They go up not down.
Also read where some guy thinks Gorilla Glue is best he had used for gluing foam board together. Must spray one side with water before mating surfaces. Here's the link
to the thread. Thought some of you might enjoy it. Good idea for building a light weight cargo hauler.