Gary, I didn’t really follow your plenum plans, but yes the original spoilboard should be flat and true. As far as the box and mounting, I used the foam gaskets sourced from Wade under the motors. Mine is setup so each motor chamber has a set of homemade check valves that allow me to use any combination of the motors. The check valves lead to a common vacuum chamber that is valved out to three sections on the table. I did use a vacuum relief valve to ensure there is enough flow to cool the vacuum motors. I did not make plans to build this system, just put it together on the fly. So far I am very pleased.
Ymmv
Cheers. Sean
Homemade check valve under the motor mount
Motor mount plate
Side view of system, the exhaust is just forced to turn 180 degrees corners a couple of times to reduce the noise