What I do is use a bunch of F-clamps and wood blocking. I heat the plastic very hot and "work it" flat, as flat as I can. It's a LOT of work, depending on how bad it is. You can wrap some blocking with parchment paper to slide along the plastic if you like. The front can be tricky.
Once it's mostly flat, I then remove it from the car (not easy if the fasteners are all rusty). I then use black 3M VHB tape, clean the leading edge of the warped plastic and then apply the VHB. Clean the bumper as well (acetone works best - get ALL dirt off and any oils). On one end, you'll leave a "tail" of release tape only, much longer than the actual plastic filler piece. Then you put it back on the car, gently tighten the nuts holding it back to the car. Don't over tighten.
now, starting from the side that has longer release tape, pull the tape on an angle toward you and apply the filler piece down - but quickly use your wood blocking and F-clamps to hold it in place for at least 24hrs. If you did a good job, it will hold permanently. Mine has been holding for about 10 years.
I can't recall which one I used - one is 25mil thick - I think I used that one, but finding short rolls might be tricky (this stuff is NOT cheap) - you can find grey stuff in most auto parts stores as it's used to hold trim, but you'll see the grey edge of it - black is nicer for both silver or black bumpers. The 3M VHB is very very strong tape!
The leading edge is now completely flat - the inboard side, well... it's as good as I could get it. Looks fine enough. One way to conceal it being warped is to use black pinstripe just above the highest warped part. It'll look less warped when it's mounted on the nose.