I dont know where you live
But the one that I built and I have the same problem (flat land)
I checked around and found a company that sells old railroad ties and they would even deliever, I started with a cup shape, with the wider side facing the shooter and stacked ties. Two high and use an auger bit drilled holes and then ran 1in pipe about 4 feet into the ground. Then as it got taller, every tie got two bars that locked the ties togther. ONce the back was about 8 feet high I filled with dirt from where they were building home near, they were happy to bring the dirt since they had to get rid of it any way.
On the back of the stack of ties I bolded 1/2 inch boler plate. bought peices from a metal shop near, mostle scrap.
From one foot off the ground to 6 feet tall it has stoped 50 cal. There are no dents in the plate.