Bags are expensive as hell and the one I want(300 lb hanging heavy bag) costs several hundreds of dollars. Never mind the cost of shipping and handling a 300 lb object. I plan on making a bag that would surpass any commercial heavy bag out there. Im talking about 400 maybe 500 lb or more in weight. I plan on doing this by filling the bag with sand and maybe even some plates at the very bottom. Obviously no ordinary fabric would be able to contain such a heavy weight while absorbing repeated abuse. So I was wondering what kind of fabric I should use. I was thinking of using Kevlar the same material used in bullet proof vests. But im not too sure on the price. My second option is nylon which is another strong fabric. What else do I need for building a bag? I heard that commercial bags contain foam in them to help absorb the shock. Where I can I buy foam?