interior rear panel damage

This panel was badly warped at the top so I heated it with a heat gun to bring it back into shape but it turns out the side also needs to be straightened.
With all the problems that I’ve been having with this set I wish I had bought repops or fixed my old ones instead of buying this used set but it’s too late now. I don’t want to heat this warp only to discover that the old warp on top has reformed. Any suggestions on how to heat things up and get everything back in line?
Should I install it on the car and then heat it?
Thanks in advance for your help