You would probably hurt yourself lifting a 300 lb sand bag, that dead, limp weight is a lot harder to carry than something solid...I actually have a 300 pound plus gravel bag made out of an old army duffel bag, not for lifting, just for conditioning leg kicks and sweeps. (Sits on the floor, I couldn't hang it if I wanted to, straps wouldn't hold and if they broke that could be the end of your foot.)
Those 70 pound tubes of sand they sell at Home Depot would be a good workout tool, they're cheap, manageable and half way durable.