People that work at Lund boats here in town use the foam they spray on the boat floors to shoot into. They spray a box full and shoot it, not sure if the blocks your talking about are quite the same, but this stuff works.
The blocks are about 10" thick. And about 12"x12". I'm thinking I'll stack them 3 wide and 3 tall then cut some plywood to surround the whole thing then a ratchet strap to hold it all together. The carpet would be a great addition. Thanks