The reason it still pops in and out is because the body line that runs down the centre of the guard - doors - quater will be out of whack from the first time it was dinted.
The best way to stop it and make it tighter is to get a bulster or something flat like a chisel ( the wider the better) and hit the bodyline out from inside the guard. It will pull the metal back into place and keep it tight.
Dont hit it so hard that the bodyline changes, all you are doing is putting it back in shape.