Lurch
One method I use is a simple jig using two x two window suckers, 2 pieces of thin alloy flat bar shaped to the contour of the guard incorporating the stretch I want, one for top and bottom
Using the same jig, but a bit more precise, space the cardboard template pieces your thinking of using every 50 or 100 mm over the length of the guard ( like ribs ) at the required reducing increments from the center working outwards to both ends EG: 50mm stretch, 40mm stretch and so on until you come back to zero, this may only be four or five pieces of cardboard each side of center, your call...
This can then be flipped to the other side of your car no problems..
Or to save the interference of the block of wood and jack you can use a good old fashioned ball bearing rolling pin and a bit of muscle once youve warmed your guard, but youll need a helper for this to apply smooth even heat over the area your stretching as you work....