[freebeard]

Somewhere near you vicinity there will be a recycler. At the very least Habitat for Humanity. In Bleugene Oregon it's BRING.

I've obtained a variety of materials there, Coroplast, double-wall greenhouse glazing, 1/8" ABS triangle cutoffs, sheet metal flashing, & etc. Your mileage may vary, but it would be less than full sheets for minimal cost. For a three-sided Kammback hood, I'd budget $50.

Sit in the driver seat and watch in your mirror while someone moves a shop rag or something around and note where the blind spots are. Then you can see what needs to be clear. ....or cut away. The transition onto any addition will be in the blind spot on the C-pillar.

It's good to have a plan. I try not to rush things too, eventual success counts.