Since you have no "proper" metal forming tools, you could form a flat piece of 18ga cold rolled steel over a welding gas cylinder, or your thigh,or something else with similar radius, any of which are "proper" metal forming tools in my shop.Its not that tight of a radius. Youd be surprised what you can do with some elbow grease and a hammer and ordinary things laying around. Just keep fiddling with it until it fits. If this still isnt possible for you, you could go to wrecking yard, find another gate, and snatch the sections either to left or right of lic plate recess to be your filler. Also from what I understand, the tailgate on an S-10 or Tahoe is the same radius as well, tho I might be mistaken. Im sure someone else will chime in with further info.