Looks like you've done some good work shaping to fit. For my truck, I went down to the local Lowe's/Home Depot and bought some strips of metal that are in an L shape but about 6 feet long. Holes about every 1 or 1 1/2 inches. You'll need some tin snips to cut and bend, but it has worked pretty well for me thus far. You may be able to mount a couple pieces from your bumper mounting locations, and bend them to give a bit of a support to the front end of the panel. I've also used 1/4" bolts to install, but Autozone has some plastic push fasteners that could work. But, following my wife's Dodge Avenger OEM belly panel install I only use 1/4" bolts at the front end.
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