have you checked out the video on a shrinking disc? here is a video link. kinda cool idea. I have one and use it for the final metal working. you just need to remember it is for the final finishing and small high spots. otherwise you can overshrink the larger section of a panel if trying to use for a bump that is too big. some guys have also used an old clothes iron to achieve the effect of heating only the high spots and then cooling that hot spot quickly so it shrinks faster than surrounding metal.
the trick with the huge felt marker is another trick I have used for years to find high spots. I personally use a long piece of plywood with no foam between the board and the sandpaper strip. this allows for a straighter panel because the sander doesn't conform to bends.
https://www.youtube.com/watch?v=IgS_zWdZz5g