Picture, finally-
This is King Starboard, aka Starboard, aka HDPE Marine Board Sheet. 1/4" thick, color is Tan or Sandshade, depending on the manufacturer. There is a gray color as well, plus black, and white.
The stuff is really soft and can be cut easily with a jigsaw/table saw/router. To mount it I used well nuts in 6 of the the original holes. 3/8" diameter well nuts from McMaster-Carr did not fit in those holes, but random mystery-sized ones from my local Ace did. No drilling or modifying needed on the actual tailgate at all.
If you live near a distributor and can pick up tan or gray HDPE at will call, it can be had pretty cheap. Around $40 for enough to match the tailgate carpet pattern. If, like me, you have to special order it from your marine supply shop, and there is a minimum sheet size, it will cost a lot more and you'll end up with a bunch of tan HDPE sitting around your garage.
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