this is my take...

drill holes in a large circular pattern on the floor in the rear... like a
manhole in size... or larger

this becomes your in-take

when parked the cool air under the vanagon replaces the hot air that leaves
from:

lewmar hatches installed behind and slightly above the rear interior lights,
both sides - it has a screen you can use to keep mosquitos out - this
becomes your out-take - about 14 inches long..

all ventilation systems need an in-take and an out-take...

in a home this would ideally be at the eaves (in), and the along the length
of the roof ridge (out)

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