I would put a deflector about six feet in front of the firing points. Set as many phone poles as you think best with a 2X6 framework on the shooter side and the downrange side, cover in plywood, close the bottom with chain link fence recycled materials could make this cost just about nothing...but leave the top open. Fill with sand/gravel and allow for drainage and replacing filler. The baffle should be 10 feet tall with six feet of shooting lane under the baffle.
Any stray high shots will go into the baffle but probably not through two plys and a foot of fill mix. Now if the firing points are raised about a foot, the berm can be reasonably lower, any round shot under the baffle will hit the berm or in front of the berm.
The police range in Long Beach California was set up this way except the shooters were in a lowered trench and ground level was bench high. The baffle was about two feet off of ground level. Behind the berms was, well a Freeway and ten bozillion folks...Use to be open to the public.
Any indoor range will have a steel (angled) shield to do the same thing. I would guess the lumber and fill would be cheaper than an extra day or two paying an earth mover.
Best Wishes on your project and retirement.
