I have to tell you, plexyglass can get pretty brittle under that hood. How about metal or fiberglass?
What kind of car? Maybe there is something in the aftermarket that you could at least start with.
My brother and I did a nice shroud on his '22 Buick using a "universal" shroud from a street rod store. It kinda looked like a big shallow cake pan. We cut it to the depth we wanted, cut the circle in the middle for the fan and bonded some brackets to it to bolt it in. We even made a piece of aluminum that covered the top tank and bonded that on as well. Came out real cool (no pun intended)