Here you can see some photos from the build of my vacuum back for Pentax 6x7:
Here is the pressure plate from behind:
The brown thing is a "wall" that was cut from a piece of linoleum. And here is the "ceiling" that was glued on the top of the wall:
Paint was used to seal the pores between the glued parts. In order to convince myself that there are no pores remaining and the air goes only through the holes drilled in the pressure plate, I put the pressure plate in a pot with water and let the air pump blow air into the device and I watched where bubbles go out.