Homasote is a gray board that is made out of paper. It comes in 4'x8' sheets and is 1/2" thick. You'll find it in almost any school being used as bulletin boards. you can push pins into it with ease. So, why on platforms? If you cover the platform with Homasote and then cover that with muslin it can be painted very nicely and it becomes very quite to walk on.
Note: We are NOT suppliers! I've gotten a few emails asking me for prices and such. Call your local lumber yard/building supplier. I don't think Home Depot has it.