With plastic bumpers or panels you really need heat to slightly soften the plastic a bit. When hit the plastic will have stretched a bit and the heat will stop the memory effect the plastic holds, ie the dent will attempt to pop back to the damaged state. Obviously the hot glue methods won't work when using heat, but the suckers will tollerate quite a bit of heat. In the past I have loosened the panel to get behind the dent, heat it from the inside to make the plastic a little bit maleable and use a soft pad to press the dent as you heat it, the paint will tollerate quite a bit of heat before bubbling, so just heat the plastic until the dent can be pressed,,,, you will feel it go when the heat is working,, different panels are of different thicknesses so they press out at different temperatures, as soon as you feel movement when pressing them out stop the heat.
