What is a punching bag filled with?
One of the coolest things about opting to fill your own bag is the fact that there are a TON of different options that you can choose from, for really cheap. Some of the top options are rolled up garbage bags (packed super tightly), mattress foam, sand,, shredded tires, even feathers and styrofoam. I was told that one of the best methods is to stuff it with ripped up clothes, rags or sheets, so that’s the way I went. My bag is about 100 lbs of old shop rags and a bit of sand.
If you go the sand route, don’t use ONLY sand, just fill the bottom enough to that it will keep the shape, then stuff line your bag with cloth, and fill the voids with down feathers or down alternative and maybe some rubber if you want it on the heavy side, if not, just opt for foam, more rags or even newspaper. Feel free to mix up your options based upon the weight that you are after, if it starts to feel too heavy lighten it with something else and vice versa. The filling totally depends on what you are looking to get out of your back, just keep in mind that it needs to be tightly packed, the rest is up to you!