from the side view drawing in your PDF, it almost looks like the two motors are already going to be close enough for direct staging where you might not even need a vent. For direct staging you cellophane tape (cellophane will melt during staging and let the sustainer free when it lights, don't use masking tape) but you might have to use a different kind of retainer for the sustainer if the screw on type gets in the way. Take a look at the
Estes Booster units, those don't have vents and are basically a variation of direct staging. You really only need vents when the gap is like more than the diameter of the motor, give or take.