My first thought was an old style external frame backpack to isolate you from the weight behind. But backpacks are all designed for distributed weight, and you are dealing with concentrated weight. A decent pack--not a cheapie--should be able to handle 30 pounds.

Try cutting up a half inch closed cell foam sleeping pad. Put a couple of pieces in the bottom of the pack to distribute the load. U-shaped, going up the front and back several inches. One of the layers could go up front and back, the second up both sides.

Try to find out what rock climbers use for packs nowadays. They tend to carry a lot of ironmongery, albeit probably not as heavy a load as yours.