My next plate experiment will be HDPE. Just took me awhile to obtain black HDPE locally. If you don't care about stealth Walmart probably has all the white material you need in the housewares dept.
17" x 12" oughta do it.
The shape of the stocker doesn't lend itself well to thick HDPE. I'd just ditch it and more or less duplicate an aluminum aftermarket skid from the material. I got lots of ideas for that, including a way to lose the "hook" arrangement at the rear and replace it with something more substantial. The stuff's kinda heavy in 3/8" or 1/2".
The working temp on HDPE is really low, something like 260 degrees. It's possible that it could be formed in a home oven if you wanted to make a thin skin. I dunno, maybe a plasics guru could tell you.
One of the reasons I wanted the skid was to use it as a form before I put it on my bro's bike.