My first back stop was made of two by tens and measured 4ftx4ftx10in and was filled with sand. That much could be be hauled in buckets, and it stopped 175gr 7mm mag/ 30-06 armor percing and a load of 30-30 bullets. sand re-fills holes where logs get channel shot throught them and bullets in time can excape

I would use 4x4's with say 2 foot sand between them so it should be say 4'x4'x2' with an open top (6'x5'x3') even better etc. You need a safe back stop so even if it take some time to fill it its worth it. A horse feed bucket or a kids small plastic pool could be used to transport sand.