The quick retrofit to existing hanging
steel targets is, again, the
addition of outward-projecting weights to the lower face, but if you
can build targets from scratch, you could use large angle bar, running
along the top of the target-plate. See the (crappy) quickie-sketch
below, which looks at the target-plate as if from the side:
In the conventional plate to the left, its center of gravity makes it
hang straight down.
In the plate with a sectional angle/bend on top (seen at right) its
center of gravity's natural desire to line up vertically with the top
hinge causes the plate to face slightly downward. Of course, you
could just simply pay/charge $$$ extra for frangible ammo, lol.