the reason a big ol dirt berm works is sheer thickness and the weight of all the dirt behind the front face, it would take unbelievable forces to push through something like a dirt burm.
that said, what you are trying to build is a containerized version of a burm, however every round you fire is going to put a hole in said container. will it stop bullets? perhaps. will it last long? no
if you can't put the dirt in the path of the bullet, then make the path of the bullet end at the dirt. or in otherwords if you can't make a burm, then get a giant piece of thick sheet steel and lay it at an angle onto some rail road ties, or logs or something, and aim within the area that creates so it knock all the bullets down and to the earth.
other than that youve basically got the options of sandbags, log piles, or if you have the money, a comercial bullet back stop for a real range in your own back yard.