Upstream of the throttle body, you would need a water manometer to measure the pressure drop- - - -just a few inches of water. The high vacuum will be found between the throttle body and the cylinder head(s). Flex plastic would be plenty rigid for the intake upstream of the throttles. You could also mock up a pipe and then cover it with fiberglass cloth and resin.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!