Yeah, I seem to remember the original Hot Rod mag was cooler than sliced bread right out of the gate (pardon the mangled metaphor).
They managed to get it right the first time, with no decades of experience behind them.
Probably the flimsiest of the clear materials readily available will be whatever is used for report-covers, sold by stationery/office supply stores.
If PTE from a soda bottle is too stiff for your application, I wouldn't even bother trying polycarbonate.
EDIT: If you want to put in the effort to get a precise fit...
Girls don't find me as attractive as they used to, and I'm pretty sure it's 'cause I quit wearing the Rolex, and I don't have a man-bun, tattoos, a murse, or a neckbeard.
Pretty much my sentiments exactly.
My temperament and instinctual interests would have been a much better fit with reality had I been a young teen at the end of WWII.
My collection of magazines and books from the post-war era allow me to experience vicariously something of a lifestyle I was too young to participate in at the time, and my modeling interests reflect much of that desire.
I find myself longing for a past I was forever out of step with, that exists now only in "historic artifacts".
I'd use PET, acrylic, or styrene for that...most likely clear PET from a soda bottle.
The only reason I recommended polycarbonate above is because he needs a big flat window that's sufficiently rigid.
The curve of this windshield gives plenty of rigidity without resorting to polycarbonate.