Are all the faceplates the same size?
If the answer is yes simply make a vacuum base out of aluminum; actually two pieces of aluminum.
Machine a labyrinth in the two pieces and fasten them together. Tap a hole for the vacuum line so it intersects the labyrinth.
Bolt the two plates securely on the machine table and machine a shallow pocket to take your faceplates.
Around the perimeter of the pocket mill a small groove to take a length of soft O-ring cord.
Drill many holes in the pocket to intersect the labyrinth between the two plates.
Put your faceplates in the pocket and apply vacuum; they are going to be held as accurately level as you machined the bottom of the pocket.
An open mind is a virtue...so long as all the common sense has not leaked out.