You will want to construct some form of baffle to keep separate the air flow from the exhaust/intake/exhaust. An indication of that consideration is clear in the orientation of the elbows currently attached.
Consider also to measure the diameters of the pipes involved, specifically at the elbows. Area equals pi times the radius squared. For the portion of the box baffled to each elbow, note the area calculated and compare it to the open area available to the box.
Just a dinner napkin guess here, but it would seem likely that your left-most elbow will have a smaller area than the gap below the planter's left side, between the feet. It is smarter to measure and compare, however.
Take into consideration how easily the gaps you will create in the planter box could be restricted by plant growth or other debris and also if the box is exposed to prevailing winds that may push the exhaust back into the area of the intake.
Depending on your final design, you may be able to direct the exhaust gaps above the intake gaps, allowing for lower cooler air to avoid the rising exhaust gas, especially if there is sufficient separation. Different sides of the box for each inlet/outlet gap may be useful, as well.
The drawing is mediocre, but may provide some assistance in understanding my suggestion. Even a thin plastic sheet material wedged between the pipes will direct and separate air flow to ensure that the intake does not pull in exhausted air.
If the exhausts are the pair at the right, you may get away with only the left most divider, as I doubt there would be any exhaust from one pipe forced into the other.
