If anybody is thinking of trying this with a throttle body
mounted where the mass air flow sensor is mounted don't do it.
It will collapse the box when the throttle is closed.
It is designed for a slide throttle mounted near the engine.

It could be beefed up to do that by making it out of
1/4 inch thick PVC foam and glass or carbon  fiber on
both sides.
Paul Lamar

That the setup I have, airflow sensor followed shortly by throttle. The
plenum and runners are made of pvc tubing segments joined with
fiberglass. Plenum is 6" dia. round pvc with a wall about 3/8" and the
four runners are 1-1/2" and 2" pvc pipe, joining a sawed-off piece of
the aluminum stock intake manifold that has the secondary injector fuel
rail.

It has worked for several hundred hours now. But I suppose knowing the
vacuum limit on the stuff would be good to know.

Perry

A six inch round plenum is a lot stronger than a 9 by 8 by 4 inch high
box.

A four inch square extruded aluminum plenum will also work fine with a TB.