I would like to see a rear 3/4 view of your car where the work would be concentrated, similar to the below concept sketch.
I took the original image from this link:
https://www.caranddriver.com/reviews...ivic-si-coupe/
I curved the lower support flanges, but they could be angled to the bumper or even come straight down for a true box shape or at lest an up side down "U".
If you surrender your truck access things will be a lot easier to affix.
Length must be based on side elevation with template overlay, do this and see how long it has to be. It may come out half as long as I've shown if you are lucky.
EDIT:
Material such as Polycarbonate or Acrylic might be out of your budget and or accessibility. Therefore
coroplast or
aluminum laminate sign board material ,scrap metal, thin plywood and other materials may be considered.
If the red plastic on the trunk lid is only a reflector and not an actual light then reflective tape on your spoiler may do.
And if as mused, an LED strip wired to the left brake light may substitute.
In my experience, the nicer you make it, and the stronger looking you make it the less likely some idiot will come along and try to damage it. If it looks frail someone may come along unable to resist temptation and test their strength on your property. Who would do such a thing? An idiot, the world is full of them.