Could you not try a valve from a piston engine inside the semi-p-port, or a sleeve(like a six port), both of which would give you that almost flush surface you would need,

Your abandoning the project, post pics of intake/carb.

 

crap forgot, piston engines with large cams have that same lack of vacuum which results in loss of braking ability and to solve the problem they install a vacuum canister, could the same be used to solve the problem with a unsteady vaccuum