This steel backstop is made very easily. First, you get
a local fabricator to cut a piece of 3/8" thick steel plate
12" by 18". Drill holes in the 4 corners for screws.
Now you get two 12" pieces of 2x4 lumber and cut a 45 degree angle on one end. Since 2x4 is actually only 3 1/2" wide, you allow for that, and cut two more pieces of 2x4 only 8 1/2" long and then cut a 45 degree angle on the end. This will now make a right angle that will support the steel plate at 45 degrees from the ground.
Screw the wood to the steel plate and paint entire unit a
nice gun gray with high quality outdoor paint.
You have an opening at the "mouth" of the trap that is
12" tall and 12" wide, save for the edges of the 2x4 wood,
which makes a very handy support upon which to staple
a target. Naturally you want to avoid shooting into
the wood, so aim carefully.
When you shoot into the steel plate, the bullets angle down
into the ground at the back end of the trap. I have shot
all kinds of loads into it with no damage. But I have NOT
used .44 magnum on it. It's possible to get a small dent
from these high powered, jacketed bullets. If so, you could
always step up to 1/2" steel plate, but I think that would
be overkill. There is a small amount of lead splatter to
the sides of the trap, and you may want to attach some
plywood triangles to the sides of the wood. It would make
it look more attractive, too.