The box chamber is a vacuum chamber which is purchasable in two basic shapes: one (called also rectangular chamber) invented for UHV applications is isolated with metal cable and flat bronze gaskets and one that act as high vacuum with a one complete side turned as an o-ring secured door.
This two versions are flat-sided so the thick walls or the extensive bracing is used to endure the atmospheric pressure.
It is your choie if you want to do one of the following process: cover multiple substrates; product degassing; clearing before impregnation, etc.