I'm going to grab a pair. the only problem I see is figuring out the volume. My thought is to seal them up and fill them with water then measure the water then convert it to cu ft.
They are very thick walls. They had to hold up to weather all across the country. I would still stick Dynamat on the inside walls. That is what the car audio guys use in cars to stop the ringing.
I've used wood chip cat litter before, and a builders bucket to find out how many litres I was adding (most have measurement gauge inside).
