Yeah, any reasonably thick piece of flat bar should work, but I am using angle for the stiffness/strength. Just going to match drill the shock holes, and have the angle go over the top of the shocks (flat) and the other leg vertical with the mounting holes on that leg. Hope that made sense.
I think the angle is a good idea. You want it to be rigid as well. I think a flat piece might just bend and try to pull the frame rails together if you get hit.
By the way, my wife had a Jeep Grand Cherokee run into the back of her car. he didn't even hit the bumper (it's a stock bumper). The entire impact was absorbed by the tailgate of the wagon. The Jeep's hood was buckled, the lights were all broken out, and it was spraying antifreeze all over the road. My wife's wagon just needed a new tailgate... the glass didn't even break!