Probably easier to find a second hand fiberglass one
Otherwise shouldn't be too hard to make out of sheetmetal or 3-4mm alloy sheet/plate. Get the edges rolled or bend them to follow the shape of the window/pillar radius. If sheet it might need some reinforcing so it wont flap around and maybe 4-6 latches to hold it on
Latches, but find some stronger similar to:

Thanks zooktastic: Has a rollbar too:

