I have made them out of bladder type water tank. You could cut the shape out of 1/4 in steel, and put it over two 4 x 4, and drive over it, and it will bend a little. After you have started the bend, put a chain around the bent steel and tighten the chain to bend it to the shape you want to weld to the back plate, as in these pictures.
You can go to a steel mill with your drawing, and they can cut and bend it to the shape your want, and then weld to the back plate.