I cut mine out then bent in the sides at the bottom at a hard angle on each side where it goes around your upper torso. then I reheated it and got my wife to bend it around my shoulders by placing it on my back while I had a sweater on, so it would mold exactly to my form.
Used a hair dryer to heat and bend it, go outside cause the fumes are pretty bad, also it takes awhile untill it becomes flexible, dont want it to turn to mush though just enough to bend it and thats it, then sit in a chair untill it hardens up.
then measure your straps on your harness and cut out your slots, about a quarter of an inch slits depending on your hooks, and about 2 inches wide, then with the hairdryer agian heat up the top of the slots insert a flat piece of metal. inside and keep working it untill you have a slight raise on the outside top part of the slits.
I made about 4 before I had a decent one. I did not add slot raises to the first 3 backplates and it did funny things because the harness straps could not angle the right way from my shoulders, the backplate was pushing them down causeing all kinds of trouble.